The rope is a new mechanic added in this game. It is a grapple rope that you can attach to particular areas and swing from. There are quite a few locations where you will be required to use the rope. You will be very vulnerable while on the rope, which makes the time it takes to get a headshot a lot more difficult, especially on the harder difficulties. You need to get 20 headshots while hanging from a rope, it sounds easy and is easy when the enemies don't do as much damage. I would suggest doing this on one of the easier difficulties.
Perform 20 headshots from the rope