Mars currently has had two rovers on the surface exploring and gathering invaluable data for the past several years, well exceeding the expected 6 month mission. Spirit and Opportunity are now showing signs of age and with a broken wheel these mechanical scientists are in need of a replacement.
NASA is taking the next step in rover tech and taking advantage of the Martian environments such as wind power. As we already have learned the Martian winds are capable of creating dust devils to sand storms. The winds have also aided the rovers from time to time by blowing off the dust from their solar panels.
The concept is mind-numbingly simple: It's a big, bouncing ball that's light enough to be pushed around Mars by nothing but the planet's wind.
Test designs have already been deployed to Martian-like environments in Antarctica and Greenland, where they successfully traversed hundreds of miles of terrain without incident. These were inflatable designs that had the ability to deflate and remain stationary (to perform experiments), before re-inflating and continuing on.