Swap

When apps need more memory than the Mac has, and compression is not enough, macOS writes some memory out to files on the startup disk. That is swap. Activity Monitor shows the total as Swap Used. The SSD is fast, but much slower than RAM, so an app whose memory was swapped out pauses while it comes back.

Swap used on its own is context, not a verdict. Memory that was swapped out and never needed again costs little. What you feel is the swap-in rate: how often the Mac reads memory back from disk. Sentinely reports both, and its measured receipts compare them before and after a fix. Swap also takes disk space, which is one place memory and Disk Housekeeper territory meet.