The idea of Google-bombing is that you get a bunch of people to write blog posts which link John McCain's name to some web page which is critical of him in some way (as I did just now).  Due to the way Google's search engine works, this tactic results in that critical web page appearing high in Google results when someone searches for McCain.

The reason this worked well in the 2006 races was that there were a lot of candidates running where the public didn't know the candidates well; therefore voters were likely to search for information on those candidates.  In the case of McCain, many people know (or think they know) him well, and are unlikely to search for him.

The idea here is to spread negative ideas about McCain using online word-of-mouth, that is, to create a kind of "buzz" around anti-McCain ideas, like "McCain wants to keep a permanent US presence in Iraq", or "McCain is a lobbyist lap-dog".  There are a variety of ways to do this, from encouraging activists to join Facebook groups with names like "McCain is a lobbyist lap dog" to getting small-audience bloggers to post entries with similar subjects, and so on.  When a progressive activist does something like that, the message only goes out to a small group of people, but those people tend to be his or her friends, and are therefore more likely to trust and believe the message.  Getting a lot of activists to do it at once helps spread the idea to more people.