This is the Timeline window, where you'll build your production. A timeline is a longitudinal view with separate tracks for the various video and audio elements you'll add to your movie.
Specifically, you'll add visual content (video, still images, and titles) to video tracks, and audio content (audio captured when you shot your video, background music, and narration) to audio tracks. Not that you'll ever need anywhere near this many, but Premiere Elements can support up to 99 audio and video tracks.