class declaration in C++

I have a Class : class RTC_EXPORT PeerConnectionInterface : public rtc::RefCountInterface { ... };

RTC_EXPORT is defined as

#ifndef RTC_BASE_SYSTEM_RTC_EXPORT_H_
#define RTC_BASE_SYSTEM_RTC_EXPORT_H_

// RTC_EXPORT is used to mark symbols as exported or imported when WebRTC is
// built or used as a shared library.
// When WebRTC is built as a static library the RTC_EXPORT macro expands to
// nothing.

#ifdef WEBRTC_ENABLE_SYMBOL_EXPORT
#ifdef WEBRTC_WIN

#ifdef WEBRTC_LIBRARY_IMPL
#define RTC_EXPORT __declspec(dllexport)  
#else
#define RTC_EXPORT __declspec(dllimport)
#endif

#else  // WEBRTC_WIN

#if __has_attribute(visibility) && defined(WEBRTC_LIBRARY_IMPL)
#define RTC_EXPORT __attribute__((visibility("default")))
#endif

#endif  // WEBRTC_WIN

#endif  // WEBRTC_ENABLE_SYMBOL_EXPORT

#ifndef RTC_EXPORT
#define RTC_EXPORT
#endif

#endif  // RTC_BASE_SYSTEM_RTC_EXPORT_H_

What does RTC_EXPORT in class RTC_EXPORT PeerConnectionInterface :public rtc::RefCountInterface {...}; do?

Usually we define classes in c++ as class Myclass{...}. What does the additional MACRO do in general?