A passive ability creates an ongoing effect that is always active while the card is in the relevant game zone. Typically this zone will be the realm, but some cards have abilities that specify what they do while in your hand, while in your cemetery, etc. King of the Realm is an example of a card with a passive ability. A passive ability is one of the few ways that an ongoing effect can be created. See the 'ongoing effect' codex entry for more details on how to handle multiple overlapping or conflicting ongoing effects.