Replenish Life is a stat found on certain items in Diablo 2. For every 10 replenish life, you will get close to 1 health restored per second.
The tiny health regeneration provided through replenish life makes it very hard to rely on the stat for healing. It may still be used for other purposes, however, such as countering the affects of poison. When combined with little or no poison resistance in the early part of the game, this may prove valuable.
The best way to heal in Diablo 2, is by using potions or auras. Using replenish life can allow you to save gold on buying potions, but it can be hard to achieve acceptable levels of health recovery, unless you switch items.
You can also combine replenish life with an aura to for faster health recovery. If you are playing as a Barbarian, then you can buy a mercenary on normal difficulty - Act 2 with the Prayer aura, which provides a small healing to your character. Very useful in early game, when combined with replenish life.
One of the easiest ways to get replenish life is from charms, and by placing skulls in socketed helms and body armor.
You can find the stat on many different items, including rings, amulets, helms and belts.