The target was the pyramid-shaped rock of unknown origin, named Jake Matijevic in honor of the Mars Rover surface operations systems chief engineer who recently passed away.
The origin and composition will be known soon: the rover examined Jake Matijevic with its Alpha Particle X-Ray Spectrometer (APXS), one of the instruments at the end of its multi-use robotic arm. Then it used its ChemCam (Chemistry and Camera instrument) to shoot laser pulses to determine the rock's chemical composition.
According to NASA, the dark, relatively smooth rock "was selected as a desirable target because it allowed the science team to compare results of the Alpha Particle X-Ray Spectrometer (APXS) instrument and the Chemistry and Camera (ChemCam) instrument, both of which provide information about the chemical elements in a target." More here.