Say you wanted to take public transportation from San Francisco Airport to Dolores Park (S.F.'s current hotspot hangout). Google maps can tell you to take the Pittsburg/Bay Point BART (Bay Area Rapid Transit) train from the airport to the 16th and Mission BART station in San Francisco. When you get off the service tells you to walk for 10 minutes to Dolores and 18th with a map showing you how.
All you have to do is go to Google Transit (google.com/transit), look at the list of cities covered (listed on page), then type in the start and destination addresses and the date you're leaving for that city.
A map then comes up with your directions via mass transit.
There's an information site with a video that tells you all about the new feature.