Random number for dice (help)

import java.util.*;
import java.lang.*;

public class Main {

public static void main(String[] args) {


    Random dice = new Random();
    int a[]=new int [7];


    for(int i = 1 ; i <=100;i++){
        ++a[1+dice.nextInt(6)];
    }
    System.out.println("Sno\t Values");

    int no;
    for(int i=1;i<a.length;i++){

        System.out.println(i+"\t"+a[i]);
    }


}
}

Can any one please explain this line "++a[1+dice.nextInt(6)]"

i know this program provides random number generated from 1-6 on how many times within the given value