It's a verbatim string literal. It means that escaping isn't applied. For instance:
string verbatim = @"foo\bar";
string regular = "foo\\bar";
Here verbatim and regular have the same contents.
It also allows multi-line contents - which can be very handy for SQL:
string select = @"
SELECT Foo
FROM Bar
WHERE Name='Baz'";
The one bit of escaping which is necessary for verbatim string literals is to get a double quote (") which you do by doubling it:
string verbatim = @"He said, ""Would you like some coffee?"" and left.";
string regular = "He said, \"Would you like some coffee?\" and left.";