The straight lines in Curiosity's zigzag track marks are Morse code for JPL, which is short for NASA's Jet Propulsion Laboratory in Pasadena, Calif., where the rover was built and the mission is managed. The "footprint" is more than an homage to the rover's builders, however. It is an important reference mark that the rover can use to drive more precisely via a system called visual odometry.
The Morse code, imprinted on all six wheels, is: .--- (J), .--. (P), and .-.. (L), as indicated in this image.
JPL manages the Mars Science Laboratory/Curiosity for NASA's Science Mission Directorate in Washington. The rover was designed, developed and assembled at JPL, a division of the California Institute of Technology in Pasadena.