Grapnel is a Physicist skill. It allows you to hang anywhere, but runs out after a given number of seconds. If it is at the max level of 5, it can be used up to 10 times.
The grapnel works by pulling you to the place where you summoned it, so if there is a wall blocking your way, you cannot cross over it.
You can move left or right to swing using the grapnel, giving you a slightly bigger overall summoning range.
You can get rid of a grapnel early by pressing ↓ or S. If you used multiple grapnels, the one summoned first will be removed.