Understanding the `-1` in the `for` loop, need detailed explanation with `for` and `if` lines of code included?

```
int[] array = { 2, 5, 1, 2, 3, 5 };

Arrays.sort(array); 

// why does this start counting from 1, and if l put 0 it goes to error, out of bounds?
for (int i = 1; i < array.length; i++) { 

    if (array[i] == array[i - 1]) {        // - 1?
        System.out.print(array[i]);
    }
}

```