When you start adding tracks to a new project, you will be able to hear all of them by default. After you have added a few tracks, you might want solo one of them to listen just to that particular track. You do this by clicking the solo button (headphone icon) in the Track header. When it's purple, it means solo is activated (1). The regions on the other tracks become greyed out, and they are temporarily muted. You can solo any number of tracks by clicking their respective solo buttons (2). When you want to hear all tracks again, deactivate all solo buttons by Ctrl/Cmd-clicking on any one of them.
If you want to mute a track, that is turn it off, simply click the mute button (crossed out speaker icon) (3). In a similar way, you can mute several tracks by clicking their respective mute buttons (4) and unmute them by Ctrl/Cmd-clicking any one of them. Sometimes muting a track that your song doesn't really need makes all the difference to a mix! :)