An array is a computer programming construct for storing contiguous data. The data can be accessed through indexing the array with the location of each element.
For most programmin languages the indices of the array start at zero. So to access the first element
The difference between starting at zero versus starting at one can be boiled down to the natural difference between counting and measuring distance. If you are counting objects, then it is natural to start at one. If you are measuring distance, it is natural to start at zero.