The file appears to be a community-made "total conversion" mod for the GoldSrc engine (the engine that powers the original Half-Life and Counter-Strike 1.6 ). What is this Pack?
Fans of these packs often praise them for how well they recreate modern visuals within the constraints of a 20-year-old engine.
A redesigned main menu and HUD to resemble the CS:GO interface. Performance & Quality
Be aware that joining community servers within these modded clients will expose your IP address to the server owner, which is standard for older multiplayer games.