Stopper knots are placed near the free end of a line. Their main purpose is to prevent the free end of a line from running through a block or clutch. They act as a brake of sorts to save the day if the wind catches a sail and, when you least expect it, an unsecured line is pulled free and is about to fly through a block or clutch.
Useful stopper knots include the figure eight and the intermediate knots. The Stevedore knot, a couple of extra half twists after a basic figure eight, is a large knot that can be used for lines that run through large blocks. I find it “too large” for utility on Skiron so I won’t demonstrate it here.