Many mobile phones nowadays are bluetooth capable, and often have it enabled by default. It's meant to be used in the same way as IR ports on mobiles: sending contact information to others and such.
Most people who own these phones, however, have no knowledge of this. Bluejacking is where you sit in a place where bluetooth enabled phones are likely to be around (cafe, train, etc) and send contacts with names like "Enjoying your coffee?". The phones generally beep as if receiving a text message and most people have absolutely no idea where the message came from: thus the fun part.
Silly name, though.