This is how to interface with a sequence in order to access subsets of the sequence. This would work, for example, on lists, strings or classes that have implemented __getitem__ and __setitem__. Knowing how to use this operation can be very powerful and useful and can prevent using unnecessary for loops.
items = [1, 2, 3, 4, 5, 6, 7]
>>> items[:]
[1, 2, 3, 4, 5, 6, 7] # all items in the list
>>> items[5:] # from the 5th index onwards
[6, 7]
>>> items[:2] # everything up till the 2nd index
[1, 2]
>>> items[:-1] # everything except the last element
[1, 2, 3, 4, 5, 6]
>>> items[-4:-1] # the last 3 elements except for the final one
[4, 5, 6]
>>> items[-3:] # the last 3 elements
[5, 6, 7][1] Python docs -- Lists
[2] PEP 204 -- Range Literals
[3] Python Tricks: A Buffet of Awesome Python Features by Dan Bader
[4] Effective Python: 90 Specific Ways to Write Better Python by Brett Slatkin
[5] Python Cookbook, Third Edition by David Beazley and Brian K. Jones
[6] Writing Idiomatic Python 3 by Jeff Knupp