As a quick glance, you have the player, comments, description, comments bar, clip info, and notifications.
You can start playing the video by pressing the spacebar or clicking the play button. If you start typing a comment, by default the video will pause so you can get your thought out and also timestamp your comment.
Once you're finished you can either press the Submit button or press Command Enter for Mac or Control Enter for PC and the video will automatically resume.
Going deeper with commenting, if you want to further communicate your idea after you have typed in your comment, select the paint brush tool and draw directly on the video with different shapes and colors.
To undo your drawing, you can use Command+Z for Mac and Control+Z for PC. Annotating does not affect the original uploaded file.
Also, if you want to leave a general comment on clips that are not associated with time, you can do this by unchecking the timecode box.