ReaLearn is a VST instrument plug-in for the Digital Audio Workstation REAPER which works in REAPER only. Its purpose is to improve REAPER's MIDI learn capabilities and add MIDI feedback (e.g. via LEDs and motorized faders). If you frequently need to map MIDI controller elements like knobs or switches to REAPER functions such as FX parameters or actions, ReaLearn might be useful for you.