Technically, a track zone is a box (width, height and length) which defines a specific Zone, each track zone in a track must be unique (that means each zone has one unique ID number).
The track zones are all treated inside the memory (virtually), that's why they look invisible inside the track while playing it. However, in order to see the track zones, you should go to MAKEITGOOD: Track zones to get what looks like what is in the picture
The track zones defines the play zone, other way said: it defines the race path or the race-able way, if some car get out of the zone, it will simply warned then may get reposition if it doesn't get again to the track zone. Also, the other usage of the track zones is, because each track zone is unique and each one has a unique ID, that's why in order to make a full lap, you need to pass through all the track zones (from 0 to LATEST_ID).