Yep. A Cunningham is a line that is used to tighten the luff of a sail. The luff is the forward part of the sail. The Cunningham attaches to some point below the sail and pulls down on it, after it is already hoisted as high as it can be (in most cases). You usually attach it to the sail by sticking a hook on the end of the the cunningham into a grommet a short distance above the bottom of the sail.