What are 'incursions' as they relate to Secret Wars (2015)?

Incursions are cosmic events in the Marvel Universe where parallel Earths collide and destroy each other to prevent the total collapse of the multiverse. The 2015 Secret Wars event is the culmination of a long-running series of incursions that ultimately led to the destruction of all universes except for one, which was then consumed by Battleworld. The Illuminati's attempts to stop these incursions were a major plot point leading up to the event.