A track is an element that contains clips. There are several kinds of tracks, the most common type is the subtitles track that contains subtitles.
Kinds of tracks
Subtitles tracks contain subtitles. It may also contain sub-tracks of any kind.
Reels tracks contain reels, which are basically movies.
Markers tracks contain markers. They are used to set marks and breaks.
Effects tracks contain effects. They are used to modify properties in all the tracks beneath.
Keyframes tracks contain keyframes. They are mainly used to store stereoscopic information.
Folders can contain sub-tracks of any kind and sub-folders. Since they are basically tracks, they can be modified and exported.
Tracks have different properties.
Each track can have its own name. It is allowed to have tracks with the same name.
Tracks can be flagged. The flag is used just to highlight the track.
A track can be marked as master.
The width specifies the size of the track in the user interface.
Collapsed tracks appear in a compact form in the user interface. Basically just to save space.
To be able to work with a track and its clips, the track must be "active".
To make it active, double-click the track's icon, or display its context menu and select "use".
A track also becomes active automatically when one of its clips becomes active.