What is c++ L"string"?

    json::value obj;

obj[L"title"] = json::value::string(U("count1"));
obj[L"num"] = json::value::number(1);


wcout << obj << endl;


// Create http post request
http_client client(U("http://localhost:3000/count"));

http_request request(methods::POST);
request.headers().add(L"Content-Type", L"application/json; charset=UTF-8");
request.headers().add(L"Content-Length", L"100");
request.headers().add(L"Host", L"testhost.com");
request.headers().add(L"X-Requested-With", L"XMLHttpRequest");
request.set_body(obj);

// Interpretation of response to request
auto resp = client.request(request).get();

wcout << U("STATUS : ") << resp.status_code() << endl;
wcout << "content-type : " << resp.headers().content_type() << endl;
wcout << resp.extract_string(true).get() << endl;