The Memory Cache service is used for temporarily storing files/objects (in memory) so that they can be provided faster for subsequent requests.
The advantage of Memory Cache over Disk Cache is that serving files/objects from memory is a lot faster than serving from disk (of course, memory (RAM) is often more limited than disk space).
Use memory caching for files/objects which are requested very frequently.
You can instantly purge (clear out) all the cached files/objects by clicking the "Purge" button on the Service status page.
See UI / Edit Service - Memory Cache for configuration options.