DARPA's out hunting for a way to launch satellites fast and on the cheap, and they're thinking that a souped-up commercial jet is the way to go to get most of (or at least some of) the way to orbit without having to rely on a ground-based launch system.
The aircraft in the picture above doesn't exist (it's DARPA, after all), but underneath the concept, you can see what looks like a missile. It would be more accurate to call it a rocket, though, and it's designed to carry a payload up, instead of a warhead down.
There are lots of advantages to an air launch system, especially from DARPA's vaguely militaristic and
paranoid forward-thinking perspective. You don't need a big ground crew or launch infrastructure. You're not tied down to a single location where weather could scrub a launch. And since pretty much everything is self-contained, launches can happen on very short notice. Like, in under a day.
By keeping things simple and flexible, DARPA is hoping to cut launch costs by up to 30%, and replace big expensive satellites with swarms of smaller and cheaper ones. Oh, and DARPA also apparently wants to make this all completely autonomous from start to finish. You've got four years to come up with something that can do the job, so get busy!