Stoned (high on marijuana) all the time. In other words, 4:20 (time to get high), 24 hours a day, 7 days a week, 365 days a year.
Mostly I've seen this as facebook updates, or graffiti, which say nothing but "420247365". I've never heard this used in a sentence.
"JM motherfuckers!" from Puff Daddy's famous Outtro to Notorious BIG's "Who Shot Ya?," which, when it was (mis)interpreted by 2Pac and Suge Knight, started the East Coast/West Coast rivalry.