Single ended dreads, also known as SE dreads, is a single dread with a loop on one end.
When it comes to installing them into your hair, you take the small section of your own hair, pull it through the loop and braid down.
Double ended dreads, also know as DE dreads, are basically 2 dreads in one. It is one dread but with two ends. When installing them you fold the dread in half and braid your own hair down the one side of the dread.