The phrase "imminent eruption" means that an eruption, usually referring to a volcanic eruption, is very likely and expected to happen very soon. Full definition
A couple links to articles on the effect of major volcanic eruptions on global climate, particularly relevant given the possible / probably / imminent eruption of Mt. Agung: (realclimate.org)
Yesterday evening, IMO noted on its website that «there are no signs of ongoing volcanic activity,» but the alert will remain red for the time being because «an imminent eruption can not be excluded.» (sciencemag.org)
Magma rising towards the surface could be a sign of an imminent eruption. (sciencedaily.com)