Simple definition
tmpfs is a temporary in-memory filesystem whose contents do not persist across reboot.
Purpose and use
tmpfs provides temporary file storage backed by system virtual memory.
How does it work?
It consumes memory according to stored contents and can use swap when configuration and memory pressure permit.
Example and verification
Monitor usage and configured limits when applications place large files there.
Points to watch
Contents do not survive loss of the mount or reboot. tmpfs does not necessarily mean storage exclusively in physical RAM.
In short
Account for both speed and volatility when choosing what data to place on tmpfs.