Mid learn is an option in Reaper to use Hardware controls on say your Music keyboard input device as controls for parameters and controls in Reaper e.g I can set my Emu Xboard keyboard rotary knobs as volume faders in Reaper for mixing and then record those changes as the song goes along if I wish much like an old Tape style mixing desk.I could set the controls to operate oscillators in say a Synth to make the sound move like using a real Mini Moog etc.
Use the Forum search function with the words midi learn and you will get more info.