Cache
A cache is data an app keeps so it can work faster next time, and can rebuild by itself if it's removed. Storage Studio clears only recognised cache folders.
A cache is data an app keeps so it can do something faster next time — web pages it has already loaded, pictures it has already drawn, code it has already prepared. If a cache is removed, the app simply rebuilds it as you use it; it may feel a little slower the first time.
Caches can grow large without you noticing, which makes them one of the safest ways to free space.
Storage Studio clears only folders that are recognisably caches inside each app's own files — it finds them by their shape, not by a fixed list of app names. It skips anything an app has open, and leaves an app's settings, sign-ins and history alone. Cache clearing runs as part of Free Up Space and Smart Care, and on its own from Clean App Caches in the More menu.
See also: Clearing app caches safely · Free Up Space · Developer caches and build folders