循环中java数组的使用

Question

counts[d1 + d2] += 1真正的作用是什么？这是我的代码：

//call simulate(10)
public static void simulate(int rolls) {
        Random rand = new Random();
        int[] counts = new int[13];

        for (int k = 0; k < rolls; k++) {
            int d1 = rand.nextInt(6) + 1;
            int d2 = rand.nextInt(6) + 1;

            System.out.println(d1+"+"+d2+"+"+"="+(d1+d2));

            counts[d1 + d2] += 1;
        }

        for (int k = 2; k <= 12; k++) {
            System.out.println(k + "'s=\t" + counts[k] + "\t" + 100.0 * counts[k]/rolls);
        }
}