The fact that this is being done in an attribute is immaterial. Using the '@' character in front of a string literal is called a verbatim string and causes any escape sequence to be ignored, allowing you to do this:
var filename = @"C:\Filename.ext"
instead of this:
var filename = "C:\\Filename.ext"
Lots of people like that because it is prettier. Resharper likes to put that '@' symbol in front of string literals.
As I recall Resharper suggests you either move localizable strings into a resource file or make them verbatim. In your case, it looks like you accepted the suggestion (either explicitly or via code cleanup) to make a string verbatim. You can turn off the verbatim string suggestion under Resharper --> Options --> Code Editing --> C# --> Context Actions --> Convert to verbatim string.
Actually, I'm NOT sure why ReSharper would have detected an Attribute constructor value as localizable since they must be compile-time constant, so it probably did that based on some other condition. A quick email to support@jetbrains.com should get that sorted out for you pretty quickly.
NOTE: The '@' symbol can also be used in front of a reserved word in order to use that as a variable name, though this is not the case in your example above:
var @string = "string";