Legendary Ability: Lair Action
Although the following describes dragon lair actions, other very powerful monsters may also have their own lair actions.
Powerful dragons' Lair Actions are, essentially, heroic deeds. A dragon can, however, only activate a single Lair Action on an opponent's turn. However, the dragon might also use another heroic deed on an opponent's turn, if the particular deed is applicable.
Adult and older dragons get Lair Actions. Exception: even a wyrmling may take a Breath Weapon Lair Action. These are powerful effects that they can enact on an opponent's turn, so they significantly impact the action economy, effectively giving a fourth action above and beyond the already significant claw, claw, and bite attacks.
Note: just because they're called Lair Actions doesn't mean a dragon can only use them when in its lair.
Lair Action Breath Weapon (wyrmling and older): a dragon's breath weapon can be used once every fourth combat round (or less often) as a Lair Action. A dragon's breath weapon inflicts an amount of damage equal to the dragon's remaining hit points on those that fail a saving throw, half otherwise. A dragon is immune to its own breath weapon. For instance, a green dragon is not harmed by the cloud of chlorine gas it produces as its breath weapon.
Lair Action Tail Swipe: on an opponent's turn, a dragon of sufficient age can make a fierce tail swipe which hits automatically, inflicting hit points of damage equal to the dragon's Hit Dice or a flat 20 damage — whichever is greater — unless a saving throw versus paralysis is successful, in which case half damage is sustained. A tail swipe may hit multiple targets that are deemed by the Dungeon Master to be in its path. For each target beyond the first, award a +1 bonus to the saving throw.
Lair Action Wing Buffet: on an opponent's turn, the dragon may execute a Wing Buffet, the wind and debris this creates inflicts 1d4 damage automatically on foes within range, regardless of their armor class or Dexterity rating. Affected creatures may make a saving throw versus paralysis to avoid being knocked prone. A character knocked prone must use up half its movement on its next turn to regain its feet, and is considered flat-footed plus an additional two point penalty to AC while prone.
Lair Action (old and ancient dragons only): Draw Upon Draconic Heritage — on an opponent's turn, as its lair action, an old or ancient dragon may draw upon its draconic heritage to remove negative status effects and regain 12d8+HD Hit Points. An old dragon may only do this twice per encounter, while an ancient dragon may call upon this thrice — though not on two consecutive rounds. The final time that a dragon uses this lair particular lair action, it forfeits any further use of lair actions in the current encounter — so this isn't likely to be done unless the dragon is sorely wounded and has decided to flee or else fight to the death.
Legendary Lair Action (old and ancient dragons only): Serpentine Rage — when a dragon invokes this lair action on an opponent's turn, then on the dragon's next combat turn, it gets Advantage on all rolls (attacks, saving throws, skill checks, etc.) A dragon may not use this particular lair action in an encounter until it has suffered injury.