A knot refers to a method of fastening or securing a line (rope) by tying. The purpose is to secure two lines together or to secure a line to an object. There are various knots for various purposes with many names such as a bowline, sheet bend, etc.
A knot is one nautical mile per hour. A piece of wood, usually triangular, is tied to a line on which knots are tied such that when thrown into the water and the line let out for a specified time, the number of knots indicates ship's speed.