Put most simply, the sheet bend is a very useful knot. It’s a very basic joining knot for tying two ropes together, and when a lot of pull is applied, it’s easier to undo than a square knot (Reef Knot). It is most always referred to as one of the most essential knots to learn, and is required to learn before a Scout can become First Class.
The sheet bend’s outstanding claim to fame is how well it holds when tying to ropes together of different sizes. For that matter, as its name implies, it was used to connect a corner of a sail to a rope by attaching the line (“sheet”) to a bight formed in the sail.
The Double Sheet Bend is most effective when used to tie a small pliable rope to a large stiff rope or to a slippery rope.
NOTE: The bottom three images in the photo montage below show the knot being tied by taking a roundturn around the back of the bight before passing the running end underneath itself. In Ashley’s Book of Knots, the correct way to tie a Double Sheet Bend is to pass the running end underneath itself twice.