Whitespaces are spaces, tabs, etc. When using the split method without any parameters, it also removes those extra characters from the end of the string. Technically, the split method creates a Python list of items from a string based on a delimiter character in the string. The default is a space. Since you only want the last item in that list (the zip code), the [-1] is added to tell Python to only return the last item in the list that split created.
If you want to learn more, start with the tutorial that is included with Python's help. IMHO it's a good place to get familiar with what Python can do as far as string manipulation.