If the feature is enabled for you (it's not for me), you'll find it in the Preferences. It lets you choose sites which you want Google to search more often (if, of course, they're relevant to your search query).
Personally, I don't see the point of the feature, and I don't like personalization of search. Right now, if your friend is looking for something online, for the most part you're able to tell him "just google xy and it'll come up as the first result." If Google messes too much with customization and personalization, everyone will have different results, and I don't see the benefit of that; I've never been a fan of recommendation engines as they've never worked well for me.
Google OS has tested it out a little bit, so you can check out how it works in practice over there.