When the character is hit by damaging attacks, his/her health is depleted. If the character's HP reached 0, he/she goes into Death-status. To restore the character's HP, one must first revive him/her from Death, then use certain items or Skills to increase it.
HP cannot go over the character's Max HP. The Max HP acts as a limit, while the character's current HP may change quite frequently, Max HP can only be increased by Leveling or Stat-Raising items.