An is a proprietary archive format used by Rockstar Games' RAGE engine. Think of it as a highly compressed .zip or .rar file specifically optimized for video games.
: If you join GTA Online with modified x64.rpf files in your game directory, Rockstar's anti-cheat system will flag your account instantly. Delete or disable your mods folder before going online. How to Fix Corrupted or Missing x64.rpf Files
(Resource File Format) is a proprietary archive file used by Rockstar Advanced Game Engine (RAGE). It contains the vast majority of game assets for the 64-bit version of GTA V, including: