Shuffle buttons game c#

I am a new programmer. I found this shuffle buttons game. I understand most of what is going on here but I don't get what is the role of "flag" and what does "i" do.. once it is i=1 but then it says "while(i<=8)".. and then what role does it have here: "a[i]". I would be grateful if someone could explain.

   namespace Shuffle_Numere
{
    public partial class Form1 : Form
    {
        int num;
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            if (button2.Text == "")
            {
                button2.Text = button1.Text;
                button1.Text = "";
            }
            if (button4.Text == "")
            {
                button4.Text = button1.Text;
                button1.Text = "";
            }
            ChekWin();
        }

        private void button2_Click(object sender, EventArgs e)
        {
            if (button1.Text == "")
            {
                button1.Text = button2.Text;
                button2.Text = "";
            }
            if (button3.Text == "")
            {
                button3.Text = button2.Text;
                button2.Text = "";
            }
            if (button5.Text == "")
            {
                button5.Text = button2.Text;
                button2.Text = "";
            }
            ChekWin();
        }

        private void button3_Click(object sender, EventArgs e)
        {
            if (button2.Text == "")
            {
                button2.Text = button3.Text;
                button3.Text = "";
            }
            if (button6.Text == "")
            {
                button6.Text = button3.Text;
                button3.Text = "";
            }
            ChekWin();
        }

        private void button4_Click(object sender, EventArgs e)
        {
            if (button1.Text == "")
            {
                button1.Text = button4.Text;
                button4.Text = "";
            }
            if (button5.Text == "")
            {
                button5.Text = button4.Text;
                button4.Text = "";
            }
            if (button7.Text == "")
            {
                button7.Text = button4.Text;
                button4.Text = "";
            }
            ChekWin();
        }

        private void button5_Click(object sender, EventArgs e)
        {
            if (button2.Text == "")
            {
                button2.Text = button5.Text;
                button5.Text = "";
            }
            if (button4.Text == "")
            {
                button4.Text = button5.Text;
                button5.Text = "";
            }
            if (button6.Text == "")
            {
                button6.Text = button5.Text;
                button5.Text = "";
            }
            if (button8.Text == "")
            {
                button8.Text = button5.Text;
                button5.Text = "";
            }
            ChekWin();
        }

        private void button6_Click(object sender, EventArgs e)
        {
            if (button3.Text == "")
            {
                button3.Text = button6.Text;
                button6.Text = "";
            }
            if (button5.Text == "")
            {
                button5.Text = button6.Text;
                button6.Text = "";
            }
            if (button9.Text == "")
            {
                button9.Text = button6.Text;
                button6.Text = "";
            }
            ChekWin();
            timer1.Enabled = true;
            timer1.Start();
        }

        private void button7_Click(object sender, EventArgs e)
        {
            if (button4.Text == "")
            {
                button4.Text = button7.Text;
                button7.Text = "";
            }
            if (button8.Text == "")
            {
                button8.Text = button7.Text;
                button7.Text = "";
            }
            ChekWin();
        }

        private void button8_Click(object sender, EventArgs e)
        {
            if (button5.Text == "")
            {
                button5.Text = button8.Text;
                button8.Text = "";
            }
            if (button7.Text == "")
            {
                button7.Text = button8.Text;
                button8.Text = "";
            }
            if (button9.Text == "")
            {
                button9.Text = button8.Text;
                button8.Text = "";
            }
            ChekWin();
            timer1.Enabled = true;
            timer1.Start();
        }

        private void button9_Click(object sender, EventArgs e)
        {
            if (button6.Text == "")
            {
                button6.Text = button9.Text;
                button9.Text = "";
            }
            if (button8.Text == "")
            {
                button8.Text = button9.Text;
                button9.Text = "";
            }
            ChekWin();
        }
        public void ChekWin()
        {
            num = num + 1;
            label2.Text = num.ToString();
            if (button1.Text == "1" && button2.Text == "2" && button3.Text == "3" && button4.Text == "4" && button5.Text == "5" && button6.Text == "6" && button7.Text == "7" && button8.Text == "8" && button9.Text == "")
            {
                if (MessageBox.Show("Congratulations! You won in "+num+" moves asnd "+timp+" seconds.", "Message text", MessageBoxButtons.YesNo) == DialogResult.Yes)
                {
                    label2.Text = "0";
                    timp = 0;
                    timer1.Stop();
                    label4.Text = "0";
                }
            }

        }

        private void button11_Click(object sender, EventArgs e)
        {
            Application.Exit();
        }
        public void Shuffle()
        {
            int i, j, Rn;
            int[] a = new int[9];
            Boolean flag = false;
            i = 1;
            do
            {
                Random rnd = new Random();
                Rn = ((rnd.Next(0, 8)) + 1);
                for (j = 1; j <= i; j++)
                {
                    if (a[j] == Rn)
                    {
                        flag = true;
                        break;
                    }
                }
                if (flag == true)
                {
                    flag = false;
                }
                else
                {
                    a[i] = Rn;
                    i = i+1;
                }
            }
            while (i <= 8);
            button1.Text =Convert.ToString(a[1]);
            button2.Text = Convert.ToString(a[2]);
            button3.Text = Convert.ToString(a[3]);
            button4.Text = Convert.ToString(a[4]);
            button5.Text = Convert.ToString(a[5]);
            button6.Text = Convert.ToString(a[6]);
            button7.Text = Convert.ToString(a[7]);
            button8.Text = Convert.ToString(a[8]);
            button9.Text = "";

            num = 0;
            label2.Text = "0";
            timer1.Stop();
            label4.Text = "0";
        }

        private void button10_Click(object sender, EventArgs e)
        {
            Shuffle();
        }

        int timp = 0;
        private void timer1_Tick(object sender, EventArgs e)
        {
            timp++;
            label4.Text = timp.ToString();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            Shuffle();
        }

    }
}