The device gives NASA the ability to use "atmospheric drag" to slow and maneuver spacecraft. Currently, slowing spacecraft requires lots of fuel and other resources. The LDSD would help astronauts conserve fuel during lengthy space travel and landing procedures.
According to NASA, the LDSD will be carried to an altitude of 120,000 feet during testing, at which point it will use rocket boosters to climb to 180,000 feet. Traveling at Mach 3.8, the LDSD will deploy its Supersonic Inflatable Aerodynamic Decelerator — a tube that will slow the vehicle to Mach 2.5. After slowing, the LDSD will deploy its Supersonic Disk Sail Parachute — a "mammoth parachute" that will carry the device to a controlled water landing. (Via NASA/JPL-Caltech)
The Los Angeles Times reports that Supersonic Inflatable Aerodynamic Decelerator is necessary to properly land a spacecraft on Mars. "The thin atmosphere can't provide substantial drag, yet it creates enough friction to burn up objects that enter it. ... This air bag will increase its area by at least 60%, creating more drag and slowing it down."
You've got to admit, it looks a lot like a flying saucer; it'd be right at home on the red planet. (Via NASA/JPL-Caltech)
While the test is meant to test how the device flies, NASA is calling the deployment of the mammoth parachute and decelerator tube a bonus, because "those landing technologies are not officially scheduled to be tested until next summer, in two additional LDSD flights."
The BBC quotes LDSD's lead researcher, who says the device will help NASA send more equipment to Mars — something that's necessary for human travel. "We're testing technologies that will enable us to land bigger payloads, much heavier payloads, at higher altitude and with more accuracy than we've ever been able to do before."
You can follow along with the test on NASA's website — check out our transcript section for a link.