From Pathfinder Wiki
Use: Also known as a Reef knot, the Square Knot is easily learned and useful for many situations. It is most commonly used to tie two lines together at the ends. This knot is used at sea in reefing and furling sails. It is used in first aid to tie off a bandage or a sling because the knot lies flat.
How to tie:
WARNING: Do not rely on this knot to hold weight in a life or death situation. It has been known to fail.