Lifegiver is a perk in FOnline that gives you +4 HP to your usually amount of HP gained every time you level-up. There are two levels to this perk, so if you take the second level of this perk, you now gain an extra +8 HP every time you level-up. This can really add up overtime to get a lot of HP.
You can first get this Perk at Level 12, and you can get the Second Level of this Perk at Level 15.
So let's say you have 100 HP and you are at level 11. When you reach level 12, you might now have 110 HP. However, if you take the Lifegiver Perk at level 12, you now have 114 HP. Once you reach level 13, you will gain 10 HP for just leveling up, and now also gain another 4 HP, making your total 128 at level 13 (114 at beginning + 10 for leveling up + 4 for having the Perk).