Glossary
Memory pressure
How hard macOS is working to make room in memory. The signal that a Mac is actually short on memory, not just using it.
A Mac using most of its memory is normal. macOS keeps recently used data in memory as cache because empty memory does nothing. Memory pressure is different: it tells you whether macOS is struggling to find room. The kernel tracks it as a level, and Activity Monitor shows it as a green, yellow, or red graph on its Memory tab.
When pressure rises, macOS compresses memory first, then writes some of it to disk as swap. That is when app switches start to lag. Sentinely uses the kernel's pressure level, together with how often the Mac reads memory back from swap, to decide whether anything is wrong. See Find out why your Mac is slow.