The file extension ARP marks replays (saved retries) of the Audition games. Audition is an online music and dance game from Korea in which you can save the performance as an ARP file for later viewing or sharing with Audition.
ARP files are usually stored in the Audition folder under 'RecData'. An ARP file may not be compatible with certain Audition versions. In addition, the song used in the ARP file must exist because it is not saved.