It is list indexing, it returns all elements [:] except the last one -1. Similar question here
For example,
>>> a = [1,2,3,4,5,6]
>>> a[:-1]
[1, 2, 3, 4, 5]
It works like this
a[start:end]
>>> a[1:2]
[2]
a[start:]
>>> a[1:]
[2, 3, 4, 5, 6]
a[:end]
Your case
>>> a = [1,2,3,4,5,6]
>>> a[:-1]
[1, 2, 3, 4, 5]
a[:]
>>> a[:]
[1, 2, 3, 4, 5, 6]