Apple - What does Purgeable means in Storage under About This Mac section?

Purgeable is a distinct concept from the "Store infrequently used files on iCloud" functionality. From Mac Help (available from the Help menu, search for "purgeable"):

“purgeable space” — or space that macOS can free up when needed by removing files from your computer. You can’t manually remove the files that are designated purgeable, but macOS removes them as space is required.

...

"Purgeable: Contains locally stored files and documents that can be removed when space is needed, and then downloaded or generated again when the files are needed again by you or your Mac."

Effectively the OS has various caches, temp files, copies of things it downloaded, etc. that are being keeping around out of view of normal users, but they can be deleted if space is needed.

If you turn on the optimized storage feature (that stores infrequently used files from Documents and Desktop folders to iCloud), they may or may not be counted with along with the system purgeable files. But even without turning on that feature, there are other purgeable files present.

These are files that the system, or you, hasn't accessed in a long time and so are stored in iCloud. If you're running low on disk space, macOS can safely delete them, keeping a 'shortcut, on your system.

When you need the file again, the system will download it for you in he background. This site has a good overview of the feature if you want to read more.

As your Mac begins to run out of space, your oldest files are quietly and automatically stored online, leaving Download icons in their places on your Mac, so that you can retrieve them if you need them. (A new graph in the Disk Utility program shows how much space you stand to retrieve this way, marked Purgeable.)

There's a post here:

Link

that makes the case that this also includes local time machine data, which makes sense given the size I see tagged as "purgeable".