Condition: Ablaze
(Ablaze is a new condition that unifies similar effects caused by some spells, monsters, and effects. If a spell or effect describes lighting a creature on fire, lighting it ablaze, igniting it, or causing it to start burning, that creature is ablaze.)
When a creature is on fire, either magical or nonmagical fire, it is ablaze.
A creature that is ablaze will take 1d10 fire damage at the start of each of its turns, unless the effect that caused the flame specifies an amount of damage instead. Some effects, like the immolation or searing smite spells, cause a different amount of damage.
Putting out the fire causing a creature to be ablaze varies in difficulty depending on what caused the fire. Nonmagical flame can be doused easily--a creature can take an action to douse the fire if the fire is within their reach, or if they have other means of dousing flame from further away, such as the control flames spell. If the flame is submerged in water or a similarly smothering fluid, it is doused automatically.
Magical flame can be more difficult to douse, depending on the source. For example, the searing smite spell can be doused as easily as any nonmagical fire but the flames from the immolation spell can only be doused by ending the spell either with a successful saving throw or an effect like dispel magic.
When in doubt, any action taken is a contest between the creature performing the action and the magical flame's originator's spellcasting DC.