What are urelements?

Some articles on urelements:

Kripke–Platek Set Theory With Urelements - Preliminaries
... Letters of the sort designate urelements, of which there may be none, whereas letters of the sort designate sets ... The letters may denote both sets and urelements ... for sets may appear on both sides of, while those for urelements may only appear on the left, i.e ...