Mac storage basics · Caches, logs and temporary files
What are Quick Look thumbnails, and can I clear them?
Short answer: A cache of file previews and icons for Finder and Quick Look. It's safe to reset with qlmanage -r cache in Terminal; thumbnails rebuild as you browse.
Finder and Quick Look keep a cache of thumbnails and previews so icons and previews appear instantly. It's usually small, though it can grow on Macs with huge media folders.
It's safe to reset: in Terminal, qlmanage -r cache clears it, and thumbnails are rebuilt as you browse. Expect icons to redraw slowly for a little while afterwards.