If something is clogged, it is blocked: stuff is stuck in it, so air, water, cars, or anything else cannot move through as it should. A drain gets clogged with hair; a street gets clogged with cars; the tiny holes in your skin get clogged with oil; the tubes that carry blood can get clogged too — doctors use that picture a lot. You can clog a thing, or it can become clogged on its own. Clogged is the form that describes the state after the blocking has happened. The word is ordinary and physical; it makes you see a blockage, not just 'full'. Unclog is the opposite action: to clear it.