The Timeline is where you synchronize the animation of your objects. When you select an object on the Stage, the corresponding object timeline is highlighted.
A gray bar indicates the global timing of a graphical object, a blue bar represents a sound object and a green bar will appear when you add a video clip or animated GIF to your animation. Per animation effect that you add, an additional sub-timeline is added for that specific effect. Your can hide these sub-timelines using the small arrow left of the object name.
To add an animation to an object you can click the little arrow next to the object name and select "choose animation". From the animation menu you can then choose a standard animation, or a specific animation if the object you selected contains such an animation.
The lines on the Timeline also represent Object Layers: when two objects visually overlap each other on the stage, the object higher on the timeline will be in front of the object lower on the timeline.
The thumbnails are miniature views of your content and are generated automatically. Thumbnails are part of the Scenes you create. By default, your content is organized in one scene. You can add additional scenes for easier management of content in your timeline: cutting, copying and pasting content to later moments in time, deleting entire scenes, inserting time, etc.
Once you’ve synchronized your objects, you can preview your animation by clicking the Play button. Click the fast-forward or rewind buttons to move forward or backward on the timeline. You can also zoom in and out on your timeline using the slider right below the Play buttons. This can help you to synchronize your content more precisely.