Rappel is a move in Shadow of the Tomb Raider that enables Lara Croft to use her Climbing Axes to move vertically without need for a ladder, or climbing a wall or ledge. She can just attach one of her Climbing Axes to a Climbable Wall and move up, down, or even swing sideways on her rope or perform a Wall Run with Axe.
There are several ways to attach to to a Craggy Wall: First, if Lara is climbing such a wall, just press one of the shoulder trigger buttons (L1/R2 for PS4, LT/RT for Xbox One). Lara will detach from the wall, only secured with a rope. This is also possible while Overhang Climbing. Another option is to jump towards a Craggy Wall and press (PS4) or (Xbox). Lara will throw her axe towards the wall and attach it there; if the craggy part of the wall is higher up than the she is, she will stay secured on her rope and can Rappel Up or Down now.