Enlarge thumbnails, and show images/videos from links with a mouse hover.
There are plenty of settings which can be customized, and reviewing them is currently the best way to learn about what the extension is capable of.
Sections on the options page, and their brief explanation:
An expandable set of rules for getting the larger image, media or other content.
It has more than 200 rules by default, which support thousands of sites across the web, including the popular ones such as Reddit/Imgur, Wikipedia (or any WikiMedia based site), deviantART, Google services (Images, YouTube, G+, Picasa...), Facebook, Twitter, Flickr...
List of user defined rules for blocking/allowing specific sites.