A file with .rep extension is a Grand Theft Auto (GTA) replay file that contains captured frame data which can be replayed in GTA. GTA is an open world car racing and campaign game that is played among players on network. REP files provide mechanism to share and analyze gameplay experience after the game is finished. Replays in such a file are teh sequence of the scenes and actions that are stored in the REP file. REP files are stored in User Files directory and can be opened with GTA Replays Manager.
REP File Format - More Information
GTA REP files are saved to disc as binary files and are not human readable. Each .rep file has a header string and contains variable number of scenes (also called frames). Any particular scene contains information about the actors, vehicles positions, weather, embroidery, etc. of any given time. As the screens are recorded one after another, they create similar effect as that of when a movie is played.
REP file format specifications are available on gta fandom and can be referred to for more informaiton about its headers and data blocks.