Items in a listbox are organized in an zero-indexed format, similar to the Python list. So, the first item resides in the first index, which is the number 0. The second item is at the number 1 ...

In Python, the most common of these data types is called the "list." The list is a zero-indexed collection of data of any type, stored linearly: the first item is at index 0, the second at index 1 ...