while having less than your maximum health your gain the benefits of several healing options.

as a bonus action or while gaining health from another source you can gain health up to 3x your proficiency bonus and your maximum hit points are reduced by half of this value. (EXP: if a 5th lvl character had 32 max HP and were currently at 15 as a bonus action you could heal for up to 3x Prof bonus, so in this case 9 your new current health would be 26 and the max would then be 28) 

maximum hit points lost in this way can only be healed by either be regained by using greater or lesser restoration (5 points per spell level), the heal spell, or a long rest witch recovers twice your proficiency bonus. 

additionally, when forced to make a death saving throw before or after seeing the roll you can chouse to reduce your maximum hit points by 3x your proficiency to heal 1 HP and continue as if you hadn't been unconscious when your turn in initiative started.