The double fisherman's knot (often shortened to "double fisherman's" or "grapevine knot") is the standard climbing knot for joining two ropes — most often used when rappelling on doubled ropes. It is essentially a fisherman's knot with two wraps in each overhand instead of one. The result is significantly more secure than the single fisherman's and acceptable for any rappel application.
The double fisherman's is also the standard knot for joining cord into a closed loop — for example, when making a cordelette or a Prusik loop from accessory cord. The knot creates a strong, low-profile join that is reliable across thousands of cycles.
Like the single fisherman's, the double fisherman's is very hard to untie after loading. This is a feature for permanent applications (cordelettes, Prusiks) and acceptable for rappels (you usually do not need to untie the join immediately). Some climbers use a "European death knot" (a flat overhand) instead for rappels that need quick joining and unjoining; the double fisherman's remains the most secure option for rope joins generally.