When a survivor is low on health, they can be too weak to fight against attacking walkers and get in a struggle. You won’t be able to use them until you save them by killing the walker with another survivor.

If all your team members get stuck in struggle, you lose the mission automatically.

There is a time limit on the struggles. If you can’t free your struggling survivor in three turns*, the walker will overpower them and you lose the mission.
*1 turn if the walker is on fire.
*0 turns if it's a spiked walker.