According to the FAQ, "your comments and the webpages you add, promote, or delete, along with the user nickname for the account you're logged in with, may be viewed by other Google searchers also in the experiment." Google mentions that this is "an experimental feature served to a random selection of participants and may be available for only a few weeks".
Google explains that the motivation behind this experiment is to test if "giving searchers increased control over their search results improves the overall user experience". Google currently personalizes the list of search results using information from a user's search history, but this experiment lets you create your own list of annotated results and share it with the world.
In a recent post from Google Blog, Amit Singhal said something very interesting:
No discussion of Google's ranking would be complete without asking the common - but misguided! :) - question: "Does Google manually edit its results?" Let me just answer that with our third philosophy: no manual intervention. In our view, the web is built by people. You are the ones creating pages and linking to pages. We are using all this human contribution through our algorithms. The final ordering of the results is decided by our algorithms using the contributions of the greater Internet community, not manually by us. We believe that the subjective judgment of any individual is, well ... subjective, and information distilled by our algorithms from the vast amount of human knowledge encoded in the web pages and their links is better than individual subjectivity.
Update. Justin Hileman has more screenshots that show some other features: you can search everyone's edits and there's a list of search edits for each user. "This whole thing looks like an experiment into crowdsourced search results," concludes Justin.
Update 2. The page where you should see your search edits: http://www.google.com/reviews/w.