You get this by saving 50 people from the hook across all games. Once someone gets downed the killer has the opportunity to put them on a hook. You will see the hooked survivor's aura and a bubble will appear around them, when they first get put on the hook. You can go over to them and pick them up off the hook using your action button (which is LB by default).
I don't recommend doing this if the killer is right on you as it is likely to speed up the other survivor's death and can potentially get you downed as well. It is usually best to wait for the killer to leave or chase after someone else.
Running the perk borrowed time can help the survivor who was on the hook have a better chance of surviving if the killer is still nearby when you unhook them. If they get hit after the perk activates it will give them a bleed out timer bar under their name. Once that runs out they will go down if they do not start healing. Borrowed time does not defend you, the unhooker, it only defends the player who just got off the hook.
Empathy is a perk that will allow you to see the auras of hurt survivors so you can know where people are in trouble and might be likely to go down and then be hooked.