The r means that the string is to be treated as a raw string, which means all escape codes will be ignored.
For an example:
'\n' will be treated as a newline character, while r'\n' will be treated as the characters \ followed by n.
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
literalr"\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.
Source: Python string literals