Pre-caching is the process by which parts of the game are loaded into your PC app to make the game ready for you to use without waiting for the whole game to download. The caching process continues while you use the game.
The PC app continuously stays one step ahead of the game and caches data files before they are required by the game. This ensures that you experience the highest quality gaming experience, along with the shortest possible launch time for the game.
When you launch a game for the first time, a progress bar appears on your PC app indicating the estimated caching time for the game.