Paging triggered on hitting low water mark - what exactly is paging?



Referring to the foll. page:

Ejection section states that “If data is continuously loaded into the bucket, its quantity eventually increases to the value indicated by the mem_low_wat watermark. At this point, paging is triggered, so that memory-availability is continuously optimized.”

What exactly is paging? Can somebody pls help understand?

On the same page, Expiry Pager section states: “Scans for items that have expired, and erases them from memory and disk; after which, a tombstone remains for a default period of 3 days.” Is this same as paging?