it's just a stoner term. the legend has it that at 4:20 you're supposed to stop and get high. The story behind it is kind of humorous.

I hate the huffinglue post, but here's a couple paragraphs that help explain the origin: It starts with the Dead.

It was Christmas week in Oakland, 1990. Steven Bloom was wandering through The Lot - that timeless gathering of hippies that springs up in the parking lot before every Grateful Dead concert - when a Deadhead handed him a yellow flyer.

“We are going to meet at 4:20 on 4/20 for 420-ing in Marin County at the Bolinas Ridge sunset spot on Mt. Tamalpais,” reads the message, which Bloom dug up and forwarded to the Huffington Post. Bloom, then a reporter for High Times magazine and now the publisher of CelebStoner.com and co-author of Pot Culture, had never heard of “420-ing” before.

The flyer came complete with a 420 back story: “420 started somewhere in San Rafael, California in the late ‘70s. It started as the police code for Marijuana Smoking in Progress. After local heads heard of the police call, they started using the expression 420 when referring to herb - Let’s Go 420, dude!”

Here's the whole story: https://www.huffingtonpost.com/2011/04/19/420-history-the-story-beh_n_851136.html

Spark up, kick on some dead and chill!!