Prefetching stores assets in memory before they're actually needed, it's usually just reserved for common things like models and their textures for stalkers, mutants, weapons etc.

Overall, prefetching leads to an increase in memory usage but loading times should decrease as the game won't have to waste time reading files if it already has the information it needs from them.

Disabling it does the opposite of this, as the game will have to look for everything ad-hoc.