Four twenty (420, 4:20 or 4/20) is slang for the consumption of cannabis, particularly around the time of 4:20. It also refers to annual cannabis related celebrations on April 20th. In locations where cannabis is legal, retailers and dispensaries will often have sales or promotions on April 20th.
Now, lets go back in time to 1971. There were 5 highschool students located in San Rafael, California that absolutely loved cannabis. These friends would meetup everyday after school at 4:20pm to go and search for an abandoned cannabis crop. They would use a map given to them by the crop grower. These 5 friends went by the names of Dave Reddix, Jeffrey Noel, Steve Capper, Mark Gravich and Larry Schwartz. They called themselves "The Waldos" because their favorite hangout spot "was a wall outside of the school." They would meet at the Louis Pasteur Statue located at the San Rafael High School after school everyday at 4:20. The term was initially called "420 Louis" with reference to their meetup spot. After failing to find the abandoned crop several times, they shortened the term to only 420. This term eventually evolved into a slang word used by teenagers with regards to consuming cannabis.
A writer for High Times Magazine popularized the origin story of "The Waldos." The first time High Time Magazine mentioned 420 smoking and the 4/20 holiday was in May of 1991. The magazine incorrectly credited the term origin to a police code. This and other false origin stories started to become more common. The connection of "The Waldos" and 420 finally came to light in December 1998. The same High Times Magazine writer previously mentioned credited the spread of the term to the followers of the band, Grateful Dead. This was after Dave Reddix became a roadie for the bands bassist, who then called for "4:20" to be the socially accepted time of day for cannabis consumption.