The Double-Fisherman Bend, which nomenclature means to "join" to of the same diameter-rope together.
It is a popular, secure (safe) load-bearing, knot that would be safe to rappel on. In contrast and for re-emphasis, the EDK knot is a simple and easy way to join two ropes together but is NOT safe to rappel on. The Double-Fisherman Bend is safe and secure to rappel on.
It should be noted that ropes made with Technora or Dyneema can be "slick" (which means you will have less friction) to rappel on and likewise goes with this knot, in order to add another margin of safety, a Triple-Fisherman Bend is recommended if you use ropes made with those components.
You will lay two rope ends next to each other and wrap around one rope twice and then using the other rope to wrap around it twice, followed by going through those wraps and pulling tightly. Then both of both of the ropes so that the knots are against each other.
Note: To do a Triple-Fisherman Bend, instead of wrapping the rope twice, you would do it three times.
Canyoneering Usage Examples:
Below, I show you three ways on how to tie the knot, via:
2) GIF (animated picture that repeats itself with no-end)
3) Video (from Canyoneering101 YouTube Channel)