This knot looks just like its name – a number 8. It is a stopper knot formed in the free end of a rope to prevent its running out through a block, sheave or fairlead. Thus it is generally used in halyard falls, sheet ends, reefing lines, kicking straps and so on.
It is used in preference to an overhand knot by seamen as it is bulkier, thus preventing the line running out through larger holes, and it does not bind so tightly, making it easier to undo when required.
Figure of Eight used in the end of a genoa sheet.