What is a slot?

Slots are time frames allocated for aircraft take-off. If your flying in Europe, they are assigned by the Eurocontrol Central Flow Management Unit (CFMU) in Brussels to alleviate air traffic and airport congestion, which may occur for all sorts of reasons, includind poor weather. Thus, Eurocontrol in Brussels is effectively in charge of all European air traffic; the airlines cannot influence slot allocation at all. Thanks to slots, aircraft take-offs can be staggered at the departure airport with a view to preventing congestion in the air or at the destination airport. Slot allocation can sometimes result in delays, with the pilot informing passengers that their flight has been help up due to airspace congestion.