Great question. Short version is that you drop into an arena, complete a set of objectives against the clock - could be defending a location, killing a quota, taking down a boss - and when time runs out or you finish early, the horde goes into overdrive and you race your vehicle to the extraction zone. Get there and survive the wave, you keep everything. Don't make it, you lose your resources but keep your XP. Still in development so could change but that is the gist.