MacOS – List all purgeable files in macOS Sierra

Download and run a free tool like Disk Inventory X or OmniDiskSweeper. It will point out where disk space is being used, by which files/folders and also allow you to delete them from the tool.

There are also paid applications that have similar features, like DaisyDisk.

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.)