Googlebomb is an attempt to influence the ranking of a given site in results returned by Google. Due to the way that Google's search algorithm works, a website will be ranked higher if the sites that link to a page all use consistent text. For example if you register lots of domains and all of them link to the main site with the text "... is a living legend" then searching for "living legend" on Google will return the main site high in the ranking.
A common means of exploiting this is through weblogs, where although the entry may disappear from the main page quickly, the short-term effects of a link can dramatically affect the ranking of a given site.