Most of the time, the Mars rover Curiosity doesn't deliver real-time information to its handlers back on Earth. For one, the data uplinks couldn't handle the constant transmission for very long. For another, the signal takes about four minutes to travel from one planet to the other. By the time an operator on Earth could see, say, an approaching Thark chieftain with his great, metal-shod 40-foot spear, said spear would have spitted the little rover and hoisted it into a low-level orbit that ended some distance away in a heap of smashed NASA hardware.
Earthbound operators program a half day or so's worth of instructions into Curiosity when its power cells awaken and allow it to start running. Then when they communicate with it again a half-day later, they receive data on what the rover has done and where it's gone. Several factors can affect the precision of some of that data, though. By counting the number of times the wheels revolved, scientists know how far the rover should have gone, but if the wheels slipped on the dusty Martian surface they may have turned more times than necessary to reach that distance. And the rover travels so slowly that distant landmarks don't change much between snapshots.
Hence the special tread of the Curiosity tires, which contain within them a sequence of short and long marks that spell out the letters "JPL" in the Morse code of 19th century telegraphy. "JPL" are the initials of the Jet Propulsion Laboratory, where Curiosity's controllers work. To see how far the rover has traveled, the scientists look at a picture of its tracks and count the number of JPL's they see, thus knowing exactly how many times the tires have rotated since the last picture.
In the event it encounters that Thark chieftain, of course, the tread will probably stop spelling out JPL (dot-dash-dash-dash, dot-dash-dash-dot, dot-dash-dot-dot) and go to a simple dot-dot-dot, dash-dash-dash, dot-dot-dot instead -- the international Morse code sign for SOS.