A track is a set of information about a single object or potential object being tracked. Objects are not confirmed as being real objects unless they are detected sufficiently often within a particular number of frames. Tracks contain information such as location history (to some extent) and estimated velocity, and potentially anything else that helps distinguish the object.
For example in theory for tracking color objects, it could contain information about the estimated mean color of the object; or it could potentially contain information about object texture information. I am not saying that it does contain that information, but that is the kind of information that could be recorded in a track to help figure out which object is which through time.
