Jackline

A jackline is a rope, wire or webbing strung from a ship's bow to stern to which a safety harness can be tethered, allowing a crewmember to move about the deck safely when there is risk of falling or being swept overboard.

At sea, falling overboard is one of the leading causes of death in boating;[1] fastening oneself to the ship with a safety harness tethered to a jackline reduces this risk.

Jacklines may be rigged temporarily when bad weather is expected, or, especially on sailboats heading offshore, they may be left in place all the time and used as necessary.

They are usually attached to strong padeye or cleat fittings at both ends of the boat, allowing the crewmember to move fore and aft by sliding their harness' clip along the line.

The reason is that flat webbing does not roll under foot while working on deck and there is less chance to confuse it with other lines in difficult conditions.

The jackline is the quarter-inch plastic-jacketed steel wire at the edge of the vessel. In this case the jackline runs from the aft starboard cleat to the bow in front of the first legs of the bow rail and back to the aft port cleat.
Blue safety line tied off to the red jackline with clip