Nothing, but quotes the following character
The backslash character ('\') serves to introduce escaped constructs as well as to quote characters that otherwise would be interpreted as unescaped constructs. Thus the expression \\ matches a single backslash and \{ matches a left brace.
It is an error to use a backslash prior to any alphabetic character that does not denote an escaped construct; these are reserved for future extensions to the regular-expression language. A backslash may be used prior to a non-alphabetic character regardless of whether that character is part of an unescaped construct.
(This paragraph is for Java programmers only.)
Backslashes within string literals in Java source
code are interpreted as required by the Java Language
Specification as either Unicode escapes or other
character escapes. It is therefore necessary to double
backslashes in string literals that represent regular
expressions to protect them from interpretation by the
Java bytecode compiler. The string literal "\b", for
example, matches a single backspace character when
interpreted as a regular expression, while "\\b" matches
a word boundary. The string literal "\(hello\)" is
illegal and leads to a compile-time error; in order to
match the string (hello) the string literal "\\(hello\\)"
must be used.