A slice bracket has three 'slots': Start, end, and step.
[2:17:3] means: Start at 2, end before 17, go in steps of 3.
[17:2:-3] means: Start at 17, end before 2, go *backward* steps of 3.
If you leave slots empty, there's a default.
[:] means: The whole thing.
[::1] means: Start at the beginning, end when it ends, walk in steps of 1 (which is the default, so you don't even need to write it).
[::-1] means: Start at the end (the minus does that for you), end when nothing's left and walk backwards by 1.