Peek means that as long as you’re touching the thumbnail, you see the zoomed in image (after a short delay). When you release, it snaps back to the front page. Without peek you would touch, view the image, and then gesture ‘back’ to go back to the main page.
This works on articles and gifs too, not just images. So if the thumbnail is informative you can save a lot of time and steps and get a quick gist of what the article is talking about