Damage. Damage is subtracted from a creature's hit points. This has no effect on its capabilities until the creature drops to 0 hit points.
If a creature has 0 hit points, see the following:
Most monsters die instantly. See here.
For a player character or special monster:
If there is damage remaining that equals or exceeds its hit point maximum, it dies instantly.
Otherwise, it falls unconscious. If it has 0 hit points at the beginning of its next turn, it starts making death saving throws.