Each search result has two buttons: "like it" and "don't like it". The first button moves the search result at the top of the page and you'll see it there every time you search for the same keywords (Google adds an orange marker so you can distinguish it from the algorithmic results). The "don't like it" button removes the search result, but only for the current query. There's also an option to add new pages that don't appear in the list of search results: "at the bottom of the search results you can give the address of a page that's relevant to your search. When you search for these same keyword(s) the page you've suggested will appear at the top with this orange marker."
All in all, you can add a list of favorites for the current query and remove the irrelevant pages. This is especially useful if you know you'll search again for the same keywords because the next time you do that the list of search results promoted at the top will certainly save you time. Google doesn't mention if your action influence the overall quality of search results, but it's likely that they only influence your search results. Google might combine the highlighted web pages to dynamically create custom search engines for specialized domains or use them to better personalize the search results.
Other similar experiments from the past included the option to add better search results, reorder the results and remove search results.
{ via Googlified and Google Discovery. The second screenshot is licensed as Creative Commons by jessamyn (some features are from the Customize Google extension). }