What does "last used" mean in the Apps window?

Short answer: Last used is how long ago you last opened each app, as macOS records it. Rarely used apps are usually the best ones to move or offload.

It's how long ago you last opened the app, as macOS records it. Apps you haven't touched in months are usually the best candidates to move or offload.

You can sort by it and filter with Older than. Some apps have no date at all — see why.