These are obsolete. They're part of the whole @ ... GET/READ mechanism that's been superseded by forms.
Just in case you have to support old code, here's the short form of what these functions do. CLEAR GETS releases the "pending" status of @ ... GETs without erasing them. CLEAR READ closes a READ, releases all its @ ... GETs and goes back to the read level above. CLEAR READ ALL closes all nested READs and fires the Valid clause of the foundation read.