A trigger can have conditions which automatically activate a Passive Ability or Reaction.
These are toggled in the character sheet or statblock by the Player Character or Game Master.
Example: When I am hit with a melee weapon attack, I deal 5 Cold damage to the Target. The Passive Ability's trigger requires me to be hit against, and I require a target for this passive ability.
(Passive abilities require no Action Point(s).)