Casablanca: Assigning a Variable to a JSON Table Value

This is probably a simple question, but I have no idea as to the answer. I've tried googling it, but I don't know what to google. I need to assign a variable to a JSON table value (username string to username table value). I saw an example on MSDN with something like this:

 obj2[L"password"] = json::value::string(U("password"));

However, if I remove the quotations and put in the variable name, it errors. I have a feeling this has to do with the "U". However, I honestly have no idea what that "U" is called (an iterator?) or how to change it. Could you help me out please?