The bowline is used to make a loop at one end of a line. It is tied with the rope’s working end also known as the “tail” or “end”. The loop may pass around or through an object during the making of the knot. The knot tightens when pulled by the standing part of the line, and has the advantage of being easily untied when no longer useful.
The bowline is commonly used in sailing small boats, for example to fasten a halyard to the head of a sail or to tie a jib sheet to a clew of a jib. You might use it to attach a line to your anchor, or put a secure loop in the end of a docking line. It is also well known as a rescue knot for such purposes as rescuing people who have fallen overboard. The victim would put the line around himself and sit in the loop, making it easier to heft him out of danger.
The bowline’s name dates to the age of sail. On a square-rigged ship, a bowline (sometimes spelled as two words, bow line) is a rope that holds the edge of a square sail towards the bow of the ship and into the wind, preventing it from being taken aback. A ship is said to be on a “taut bowline” when these lines are made as tightly as possible in order to sail close-hauled to the wind.