This is one of the most basic knots. It is the low-tech of knots. It is the building block of many other, more complex knots.

Used by itself, the overhand knot is a stopper: a thicker feature in the rope that prevents it from slipping through a narrow opening, like a hole in an object or even a tight loop in different knot, part of the same or different rope. It is very stable, even becoming jammed, extremely hard to untie. That is clearly very helpful when used for its other common function: to tie up the ends of the rope, preventing them from unraveling.