A track is a linear representation of animation occurring over time. You can think of a track as a long, straight railroad track, with the animation start time at one end, and the finish time at the other. Each animation appears on the track at the time of its occurrence in the animation. Every item in the hierarchy has a track that shows how the item changes over time.
Range track displayed in track bar below time slider
The values in an animation track are usually displayed as keys. Some controllers don't use keys and instead display their values as a range bar or some other graphic symbol. For example, the Wave Form item displays a sound file as a two-channel sound wave.
In the Dope Sheet, range tracks indicate when items below the track are animated. A range bar in the track displays the frame range of time over which the animation occurs. You can edit the range by dragging the track or its endpoints.