When you take damage from a creature you can see within 60 feet, you can take a Reaction to transfer some of that damage to a creature that is the sole target of your Hex. You take half the damage (round down) and the hexed creature takes the remaining damage.
Once you use this feature, you can't use it again until you finish a Short or Long Rest. You can also restore your use of it by expending a level 2+ spell slot (no action required).