The drone needs to carry a "powerful" embedded computer, a camera and an inertial measurement sensor on its back (shown above), but it's effective -- the drone can practice as much as necessary without smacking into something real. The results aren't perfect when the drone still ran into trouble six times out of 119 real-world flights, but that beats crashing relatively early.
While safety is clearly important here, the training environment is ultimately focused on speed -- the team took its inspiration from drone racing, where pilots frequently have just a split second to dodge fellow fliers or unexpected hurdles. However, its applications go well beyond that. Researchers imagine this helping drones learn to fly real-world buildings at speed, or even to dodge around moving humans. This could be useful for everything from rescue missions to the battlefield, or even just follow-me sports drones that could stay close to a fast-moving human without fear of a collision.