This is a plugin you should only install if you want to use it with a NI Komplete Kontrol Mk2 series keyboard. Here is the main thread explaining what ReaKontrol does:
https://forum.cockos.com/showthread.php?t=222331
It may cause problems with non supported NI keyboards which use the same MIDI device name depending on your configuration for that DAW MIDI port (e.g. if you are also using this port for MCU transport control of your NI keyboard with Reaper). Simply delete the file if you have installed it by accident (possibly installed via ReaPack?).
Or - if you have a compatible keyboard - you can follow the installation* instructions provided with the plugin and enjoy a lot of features on top of just transport control.
*Specifically, make sure to
disable the Komplete Kontrol DAW MIDI device for ReaKontrol to work properly: Options>Preferences>Audio>MIDI Devices: The Device “Komplete Kontrol DAW - 1” (S-series) or “Komplete Kontrol A DAW” or “Komplete Kontrol M DAW” (A/M-Series) should be disabled as input and output. Leave your normal Komplete Kontrol MIDI Device enabled.