What does "::" mean?

[qwweeeit at yahoo.it]

Hi Robert,
I didn't succeed in reversing a string with the "full form" you
proposed:
live[len(live)-1:-1:-1]     # where live="live"
The result is an empty string.
To  reverse "live" (in a "full form"), I have to put a char in front of
the string and...:
('x'+live)[len(live)+1:0:-1]   # --> "evil"
Is it due to the Python's version (I still have 2.3.4)?
Bye.