Timeline shows all objects within the currently selected page. Sound which does not attach to a button will also show in Timeline. Timeline is very useful to easily adjust the object timing within a scene.
When the movie is previewed, the timeline will show a play cursor, showing the frame currently is playing. It is a very handy feature to visually see what object is supposed to show at a specific frame.
Adjusting object timing
By simply dragging the end of the object bar you will adjust its duration. Drag the start of the object bar to adjust when the object should show/play while dragging the middle of the object bar to move the show time and duration at once.
Adjusting object Entry/Exit effect duration
For object which has an entry/exit effect, you will see a triangle on the left and right of the object bar. This triangle represent the Entry/Exit effect duration. You can adjust the duration by dragging the start and end of this triangle.
Adjusting scene duration
You can also adjust the scene duration by dragging the red vertical bar at the top of the timeline. Note that you cannot drag the scene duration less than the object duration within the scene.
Making scene to pause/rewind/go to the next page at the end of its duration
To make the scene pause at the end/rewind/go to the next page, click on the small green play button next to the red vertical bar. Each click will toggle from one mode to the next.
Note that when the scene is paused at the end of the playback, all objects will also be paused with it except objects which are set to Hide at start and a non synchronized sound. These type of objects will keep playing.
Selecting a special object (eg: sound and layer effect)
Click on the object bar to select the object. This is useful to select a sound object which does not have any visual representation in the Preview window.
Adding a pause at a specific time
To add a pause at a specific time, click on the Timeline ruler. A pause icon will appear. Click it again to removes it.
Note that when the scene is paused, all objects will also be paused with it except objects which are set to Hide at start and a non synchronized sound. These type of objects will keep playing.
Changing Timeline display settings
You can change the timeline display settings by make it narrow/tall, showing object name, duration, enabling/disabling synchronized sound waveform. Click on 
at the lower left corner of Timeline window to do so.
Note that only a synchronized sound will show a waveform. There is no point of seeing a waveform for a non synchronized sound as upon playback the sound timing will never be synchronized to the object timing. Sound waveform is very useful to match the object timing with a specific part of your synchronized sound. Eg: Normally you play the movie and hear the sound and see where it correspond to a particular bump in the sound waveform, then you can adjust other object Show/Exit time to match with the particular bump to make sure they are happening at the same time upon playback.