3 edit mode
Clean Up is used to clean up a mesh and remove garbage as it often accumulates while mesh modeling.
It's usually run fairly regularly.
No selection is required, but note that the tool works on all current edit mode meshes
Using Clean Up
This video demonstrates an earlier version of the Clean Up tool, with some of the current features not shown, but the operator properties should be self-explanatory.
In recent versions, the Cleanup tool features a fading HUD, informing you what, if any, geometry has been removed.