The timeline is the section at the bottom of the screen. It's made up of several horizontal tracks where you can arrange items to form your movie. These items include anything that would be featured in your movie, from video clips to images, Powerpoint slides, PDFs, even audio files. The tracks run from right to left, and the items can be arranged simply by dragging them.
Items on higher tracks will appear as the top layer in the video. In the screenshot below, there are three items on the timeline. These are similar to Photoshop's layers. Because the title card (listed in the image as "Callout") is on the track above the image of the library homepage, so it will appear onscreen.
The playhead, circled below, allows you to select an exact point on the timeline. Whichever point is selected is highlighted in the canvas (currently showing the library homepage).
By clicking on the green and red tabs at the top of the playhead, you can expand the selection to include a ranged selection.