The tool is a motorized, wire-bristle brush designed to prepare selected rock surfaces for enhanced inspection by the rover's science instruments. It is built into the turret at the end of the rover's arm. In particular, the Alpha Particle X-ray Spectrometer and the Mars Hand Lens Imager, which share the turret with the brush and the rover's hammering drill, can gain information after dust removal that would not be accessible from a dust-blanketed rock.
On August 5 of this year, NASA will be sending the rover Curiosity to Mars. Or at least they'll try, as this shows us the hundreds of things that need to go right for a rover to land on the surface of Earth's red cousin.