Memorizing a few useful knots can help you deal with a variety of tasks, ranging from rigging a ridgeline for your survival shelter to simple tasks around the house. If you're carrying cordage, you need to know how to use it effectively or it won't do you much good. Fortunately, even if you only know a handful of knots, you can learn to make do.
One situation you've probably run into is dealing with scraps of paracord or rope. Tying together two loose ends can extend your cordage, but if done incorrectly, these ends may slip or loosen over time. This is where the double fisherman's knot comes in handy.
The double fisherman's knot is an extremely secure way of connecting two pieces of cordage. Each end of the cord is looped around the other twice and tied off, so when the ropes are pulled apart, the knots slide together and cinch down. The following video from Paracord Guild shows how it's done:
For added security, the ropes can be looped three or four times before tying — this would create a triple or quadruple fisherman's knot.