These are parity files, used to reconstruct any missing parts of the archive. Ordinarily you will not have to do anything with them -- they are extraneous unless a part is missing or bad, in which case the torrent's creator should have fixed the archive before distributing the torrent. If WinRAR does give you a message about a missing or corrupt part, then get SmartPAR (Windows) and open the .PAR file. The program will then check all the files and recreate any missing or damaged parts. For OS X, UnRarX should also process the PAR file.