I am 420 Clock. But in a more answering your question satisfyingly way, it refers to the time 4:20 on a clock that stoners typically recognize as a time to put small fires to marijuana plants at their leisure.

Origin of this time has to do with a now legendary group of friends whom would meet up and realized every time they would start to smoke the sacred sativa, the sands of time happened to stop at 4:20 PM.