The Harvest project is a collection of files that are useful to people using the Reaper DAW, such as preset lists and note maps.
A MIDI drum map is a list of names for MIDI notes. Whereas pitched instruments share common note names such as C sharp or B flat, atonal percussive instruments often create a completely different timbre for each MIDI note. These maps keep track of which drum sound is triggered by playing which note.
A ReaBank file is a list of synthesiser presets (banks and programs) for Reaper. It allows you to easily bring up a synthesiser patch directly via the DAW.