It's the 42nd mission to the International Space Station, and since that number has significance for geeks all over the known universe, the crew, likely fairly geeky themselves, made this wonderful poster to commemorate their mission.
The crew is parodying the poster for the movie Hitchiker's Guide to the Galaxy, which, of course, is based on the series of books by the same name, and in those books the number 42 is revealed to be the answer to the question of life, the universe, and everything.
Also worth noting: this may be the first crew mission poster to include GM's Robonaut so prominently, and if you look at the little silhouette of the space station just above the title, you can see the lower left solar array is turned edge-on to the viewer, leaving the other three to form what looks like a '4.' The other side's arrays are all facing head on, forming a roman numeral "II." So, 42. At least I think that's what they're doing.
Just a reminder that astronauts and cosmonauts are great.