It depends on which system you are using. On windows Reaper usually uses VLC as first option decoder for video. Its meant to play every video in Reaper that also can be played in VLC.
You also can install (download) ffmpeg for decoding. But this needs to be adressed in the decoder priority list. (preferences/media/video). Right next to this list is also the "Show available decoder" button which should help you to determine if you even have a working decoder on your system and where it is located.
