What kind of reflection does Unreal use?

I know for a fact that unreal uses a reflection system in order to have C++ scripts. I also know that it uses macros in order to define how the variables should be reflected by the engine. Here comes what i don't know:

Are this macros defining metadata from within the code execution (filling lists, defining structs, etc...), or are they just markers for a parser that generates more C++ code, which then is used by the engine to know which variables to reflect?

It has to be one of this two, since I highly doubt that macros could help with parsing debug code, which is the only third option that I have found after investigating several C++ reflection systems.