It's the offset of the first array element from the start of the array. The first element is at the start of the array, so the offset is naturally zero—there are zero elements prior to the first element. Like the markings on a ruler, array indices are cardinal numerals (0, 1, 2, …) counting the distance from a reference point, not ordinal numerals (1st, 2nd, 3rd, …) representing rank in a sequential order.

If you're comparing arrays to centimeters on a ruler, the first element is at the center of the first cell; at zero, there's the left border of the first cell.

(Also please note that both this and my previous reply are tongue-in-cheek).