A jackline is a piece of safety equipment on a boat that sailors can use to reduce the risk of falling overboard. It consists of a length of rigging on both sides of the boat running from bow to stern. Sailors can attach themselves to the jackline with a safety harness when conditions are poor and there are concerns about falls. If a sailor does lose her balance and fall, the jackline will prevent her from going into the water and will make it easier to get back up onto the boat.
Working on deck can be dangerous, even in good weather conditions. People may fall overboard if the boat pitches unexpectedly or they lose their balance while performing a complex task. There's also a risk of slipping on objects left on the deck, or on patches of algae, ice, or other hazards. In bad conditions, the number of sailors on deck is limited for safety, and they may need to wear harnesses to make sure they will be safe in the event of a fall.
The traditional jackline was a rope or length of steel cable, running between cleats mounted on the deck. One problem with this design was that if sailors stepped on the line, it would roll, and this could imbalance them. Most boats prefer to use flat nylon webbing, which lies flush against the deck or crushes under a sailor's foot, rather than rolling. This is much safer, and is just as easy to use as a cable or rope.
Some boats may leave the jackline in place at all times, although sailors may not be required to use it unless the weather is bad. This can be easier than trying to rig the line in poor conditions and makes it very easy for sailors to follow safety protocols while on board. Sailors on other boats may stow it when it is not actively needed, and bring it out for hazardous weather warnings or when they are taking the boat out on the open ocean. In both cases, sailors must regularly check the line to make sure it is in good shape, checking for frays, tears, and other problems.
To use the jackline, a sailor puts on a safety harness and makes sure it is properly fitted. A line from the harness can be clipped to the line with a carabiner or similar device. The sailor should test the line to make sure it is sound before moving about the deck.