To generate a new track, you simply select a genre, activity and mood that matches the tone of your video and click "Play". Every time you click "Play", a new unique track is produced for you to preview. This is what is known as "generating a track". Not every track you generate, you will want to add to your video to download. You'll want to generate a few to find the perfect-match for your video.
When you find a track you're happy with, simply save it and click 'Add to Video'. Here, you can paste in any YouTube URL (or upload a video) and you'll be able to merge your track with this very same video. When you're happy with the volume customizations, click "Create". Your video will now be merged with your brand new Infinitunes audio track and available for download. This is what is known as 'Download Tracks'.