This classic knot is useful in situations where you're connecting two pieces of rope. You can always add more pieces of rope, and this knot can be used almost anywhere, especially when you need to tie something loosely to your pack, or make extra line for a tarp or rain fly. Bonus: It's easy to untie, even with a lot of stress added to it.
STEP BY STEP: Holding the two ropes together at one end, take the left strand under the right and under, then right over left and under. Take the left part of the rope, put it over the right and wrap around. Take the new right side of the rope, put it over the left end and wrap it under.