A jackline is a line or strap used 0n a sailboat to help keep you on the boat. Typically a jackline is run from the stern to the bow on both sides of the boat. A sailor wearing a safety harness uses a tether to connect to the jackline when moving along the deck of the boat. Having one continuous jackline from stern to bow makes it easy and safe to stay clipped on the whole time when moving forward to the bow from the cockpit.
Shown here is a commercially available jackline coiled before use. Typically sailors stow jacklines away until needed or until taking the boat offshore, when it’s a good idea to have jacklines in place so that they’re ready when needed.
When purchasing a jackline, get one about the length of your boat. Typically a jackline runs from a strong cleat at the bow to one on the stern—one on each side.