If something's afloat, it's drifting on the water. When you sail toy boats, you set them afloat across a pond or lake.
You can set something afloat, like an inner tube in a pool or a leaf on a stream, or you can struggle to stay afloat, as when the passengers of a capsized boat try to keep their heads above water. In either case, being afloat means floating on the surface. The Old English word is aflote, from a Germanic root word.