Arrays in Java work with zero based indexing, meaning that elements are indexed 0, 1, 2, 3, 4, 5. In your dice program, your dice are numbered using one based indexing: 1, 2, 3, 4, 5, 6. The "-1" is necessary to convert from one based indexing to zero based indexing. For example:
If the human player played a six (6), then one must be added to the sixth element in the humanCount array. If you use six as an index you will see that an Exception is thrown that says that you are trying to access an element outside the bounds of an array. So, one is subtracted to give 5, which is the last element of the array.
Anything else?
