Spider webs are traps that can be found in World 4. Spider webs are sticky and Mos will be trapped. Repeated jumping will free her. Spider webs are non lethal but they might slow down your pace or let another enemy catch you. Beside being a trap spider webs can be used to reach higher ground.
For speedruns it important to know that you can bypass a spider web by holding the jump button even after landing on the ground.