r means the string will be treated as raw string.
From here:
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.
