Python - what does an 'r' before string mean?

It means that the string will be treated as a raw string.

You can read about the raw strings here, in the Python documentation.

When an 'r' or 'R' prefix is present, a character following a backslash is included in 
the string without change, and all backslashes are left in the string. For example, the string 
literal r"\n" consists of two characters: a backslash and a lowercase 'n'. String quotes 
can be escaped with a backslash, but the backslash remains in the string; 
for example, r"\"" is a valid string literal consisting of two characters: 
a backslash and a double quote; r"\" is not a valid string literal 
(even a raw string cannot end in an odd number of backslashes). 
Specifically, a raw string cannot end in a single backslash (since the backslash 
would escape the following quote character). Note also that a single backslash followed 
by a newline is interpreted as those two characters as part of the string, 
not as a line continuation.