When the Keepers or their A.I. refer to an 'Event', they are referring to any mass extinction event a planet might suffer. Because their sensory and predictive models are so powerful, they can generally pinpoint such Events long before they take place, ensuring they are able to rescue representatives of any creatures that might be impacted by the event. They typically limit their rescues to two (one male and one female) of a given species for logistical reasons.