In the Preview Window, you can play the video that you are working on in the timeline and preview the editing you've done so far.
Here is an introduction of all buttons in the Preview Window.
Playback/pause: Click the button as the video shows to start or stop playing a media file.
Size: Adjust the size of a video in the Preview Window. By default, it is set to Fit, which make the videos completely fill the Preview Window, while you may select a percentage(20%, 50%, 200%, 300%) to zoom in/out the videos.
Definition: There are 3 options for preview definition: Smooth, HD, Hyper-Lucid.
Go to previous/next frame: When you click the Previous frame button or Next frame button once, the cursor in the timeline will move to the last or next frame.
Go to previous/next edit: When you click the Previous edit button or Next edit button once, the cursor in the timeline will move to the end or the beginning of the last or next file.
Full: This will enable you to play files in the Preview window in full screen.