With each level of this Perk, you gain an additional 4 Hit Points every time you advance a level. This is in addition to the Hit Points you already gain per level based off of your Endurance.
”— Fallout 2 In-game description
Every time your character gains a level, you gain an additional four Hit Points for each rank of this perk. With two ranks of Lifegiver, that's +8 HP each level, which is more than 10 points in Endurance would give you.
Although it's not clear from the description, Lifegiver grants +4 HP for every level gained, including the level when the perk was taken.
In the data files, it is listed as being available in 3 ranks, and loading screens will mention multiple "levels" of the perk.
The corresponding image is a Vault Boy version of Jesus.
Considering the relatively small amount of HP it provides, players might want to consider Intense Training rather than taking this perk if they have less than 9 Endurance. A single point in Endurance would provide 20 HP, +2 Survival/Unarmed, minor poison/radiation resistance, and access to another Implant.