Mac storage basics · Memory, swap and speed
What are swap files on a Mac, and why do they use disk space?
Short answer: Swap files hold memory macOS has moved to disk when RAM is full. They count as System Data, show as Swap Used in Activity Monitor, and clear on restart.
When apps need more memory than your Mac has, macOS compresses memory and, if needed, moves some of it to disk in swap files. They live in a hidden system volume and are counted in System Data. You can see how much is in use on the Memory tab of Activity Monitor (Swap Used).
Swap is normal in small amounts. Large, constant swap means memory is over-stretched — quitting heavy apps or browser tabs helps — and a restart clears the files. Don't try to delete them by hand.