The Track menu explained

Add
Creates a blank track window. Be default new tracks are added after the currently selected track. You can change this by enabling the 'Add new tracks at the end' option (read about it here).

Delete
Let's you delete the currently selected track. By default a window asking for your confirmation will appear. If you want to disable it (you are completely sure what you are doing everytime) just select the 'Don't ask before deleting a track' option (read about it here). The header is deleted automatically when you delete the last remaining track of the cuesheet.

Apply
Saves any changes made to a track or cue header. If no track is selected or you are editing a new track (after pressing 'Add') it adds a new track to the cuesheet.

Next
Lets you select the next track in the cuesheet window.

Previous
Lets you select the previous track or the cue header in the cuesheet window.