It's part of their
SearchWiki initiative. You have to be logged in to a Google Account in order to see the up and down arrows.
The votes will only affect your own search results, although any notes you might choose to add will potentially be visible to other searchers.
Eventually, if they gather enough data points, it's possible they might use the voting data in general search in some way, but that's sheer speculation at this point. According to Google, "The changes you make
only affect
your own searches." (Read the Official Google Blog entry I linked to above for more information. There's a quick and informative video there that walks you through the features.)
It might be a good idea to periodically log in to your Google Account and check the SearchWiki notes associated with important target phrases for your company site just to see what customers might be saying about you (and to make sure competitors aren't sneaking in and trying to badmouth you behind your back).
--Torka