Normally, once a monster's Hit Points reaches zero, that monster is defeated. However, some monsters have multiple lives that must be depleted before they are truly defeated. In this case, being reduced to zero HP simply lowers their life count by 1, with their Hit Points being restored to maximum (typically 30,000). As such, every monster that has over 32,000 HP has two or more lives.
No matter how much damage is done to one life, it does not carry over to the next. For many superbosses, this means that only damage in intervals of 30,000 matters.
On the flip side, however, once a life is depleted, healing cannot restore it. This is critical for chipping away at certain superbosses that frequently heal for large amounts. As their Hit Points cannot go above 30,000, most of the time their healing has no effect.
Note: There are a few monsters that can restore their lost lives. They cannot go above their initial starting lives, however.