When Living Seed was first introduced, it was broken. I think a lot of people believe it is still broken, but I disagree. It was fixed slightly (I believe in 3.03). Upon critting with Regrowth, HT, Nourish, or Swiftmend, a Living Seed (buff) procs on the target that is equal to 30% of the effective amount healed. The Living Seed buff applies an Earth Shield-like effect. If the target takes an additional hit while Living Seed is on them, Living Seed will mitigate or "shield" a portion (or all) of that damage.
Let's say a tank is down 4k hp and a druid casts Regrowth, critting the tank for 8k. Since the effective amount healed was only 4k, the Living Seed will proc for 30% of the 4k, giving the tank a 1.2k Living Seed buff. If a boss hits the tank with a 10k swing, the Living Seed will proc, mitigating 1.2k of that 10k damage, and the tank will only take 8.8k worth of damage.