An ability is the special game text a card contributes to the game.
Unless an ability explicitly references an out-of-play area (such as a hand, deck, archives, or discard pile), that ability can only interact with cards that are in play.
Ability Types:
Mack the Knife has both an Action ability (Deal 1 damage...) and two Constant abilities (Elusive and You may use Mack the Knife as if it belonged to the active house)
Mighty Javelin has the Omni ability: Sacrifice Mighty Javelin, Deal 4 damage to a creature
Horseman of Pestilence has Play, Fight, and Reap abilities. Deal 1 damage to each non-Horseman creature.