One of the many ropes
terms) on a Laser sailboat
, as well as some others.
Unlike other boats, the main sail is not threaded into the mast, so a Cunningham ties onto the corner of the sail where the mast and boom meet, and onto the bow of the boat. The rope is threaded through a eye hole and onto a 1-way cleat block, so it can be tightened while sailing. This is tightened to keep the sail taut and reduce luff.
Also, when (not if) you capsize, the cunningham also keeps the mast pressed down on the boat, so it doesn't fall out.