Tracking means how well the device is able to detect the correct tone of the guitar.
It is most often used in connection with pitch shifters, harmonizers and octavers. These devices detects the pitch of the guitar's string and adds another signal to it (or pitches it up or down). If the pitch is detected perfectly, the generated signal will be in perfect harmony with the original sound from the guitar. If the tracking is off, the device may add a signal that doesn't harmonize with the original sound.
one big problem that can throw tracking off is being out of tune. even slightly. also if you put too many effects in front of the tracking device, it may be off a bit. its also recomended that you have high action for best results.