Hummingbird is about synonyms but also about context. Google always had synonyms, but with Hummingbird it is also able to judge context - thereby judging the intent of a person carrying out a search, to determine what they are trying to find out.
Hummingbird name was derived from the speed and accuracy of the hummingbird. Hummingbird places greater emphasis on natural language queries, considering context and meaning over individual keywords.