Your health regenerates before each battle, but health potions aren't as effective.
After a recent patch, as opposed to healing your full health this perk will now heal you for half of your maximum HP at the start of each battle. While not insanely overpowered, this perk is still very worthwhile for a character with 200 or more maximum health.
Health potions will heal for half as much (5 hp as opposed to the usual 10).
It affects the Druid and Bloodlust perks by halving the total health you would have gained, rounded down (effectively halves their level).