It means marijuana, it came from the police code for posession of it, which is 420. It has transformed into a time of day that stoners take heed to and smoke to celebrate that 4:20 has come around. April 20 is also known as the international get high off your ass day. Coincidently, it is the same day as: the columbine shootings, hitler's birthday and earth day.
1. Rodger that, we've got a 420 down on Mulhand drive. Need backup.
2. Somewhere its 4:20. (Lights a bowl)
3. Only a week till 4-20, I'm excited.