A plugin is a library, in *.dll format (for Windows) that emulates a certain component of the N64, like the graphics processor OR RSP. There is a large number of different N64 emulator plugins, and they all should work with Mupen64 just fine.
You can get a large number of Nintendo 64 emulator plugins from the download section of this forum. There is no "best" video, sound, RSP or input plugin, as they all have different strengths & weaknesses, so the only way to get a decent plugin is through experimentation to see which combination of plugins works the best.