How does .IndexOf()? works here?

what is the use of == -1 in this code? i can understand that adds 20 random numbers to an array but i can't understand why is it there? debug in Visual Studio didn't help to understand why. removing it returns can not implicitly convert "int" to "bool"

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   int[] Numbers = new int[20];
        var counter = 0;
        do
        {
            Random random = new Random();
            var randomNumber = random.Next(1, 100);
            //what is == -1?
            if (Array.IndexOf(Numbers, randomNumber) == -1)
            {                                               
                Numbers[counter] = randomNumber;            
                counter++;
            }
        } while (counter < 20);