A cliff is a steep and often sheer rock face. If you stand on the edge of a cliff and peer over, you'll be able to see the waves crashing on the rocks below. But if you don't watch your step, you could fall right off that cliff.
So many action movies feature a scene in which a car chase leads to a car full of villains plunging spectacularly off a steep rock wall, or cliff. As the car hurtles over the edge of the cliff and smashes onto the rocks below, it invariably explodes in a shower of flames and a cloud of smoke.