Here's how Google describes the "What's Popular" gadget:
The What's Popular gadget by Google helps you discover interesting items from all over the Internet.
How we find interesting stuff
What's Popular uses algorithms to find interesting content from a combination of your submissions and trends in aggregated user activity across a variety of Google services, like YouTube and Google Reader.
You can rate items that you like or dislike by clicking the ratings buttons next to each item. If you change your mind, you can change your rating at anytime. The What's Popular gadget looks at what users liked or disliked to improve its ranking of interesting items.
Add a URL
Tell What's Popular about interesting items you have found! If they become popular, they may rise to the top of our list! You can submit an item with or without attribution to yourself and preview your submission before you post. If you decide to associate your username with your submission you'll be able to edit its title and description. The items you submitted can be found in the "My adds" area, where you can remove your name if you change your mind.
The gadget is a part of a service that will be released in Google Labs, where it's already hosted. Right now, the most popular web page has 13 pops (probably a name for upvotes), there are three categories: stories, videos, images and the pages are sorted by relevancy, the algorithm favoring recent content.