Basically, a way to say "let's go smoke some pot".
This originated from California's San Rafael High School, where there was a club that participated in marijuana (weed) smoking every day at precisely 4:20
Peter: Cmon kid school's over, lets go
Daniel: yuss it's time for us to 420 blaze it
Basically how to say "cmon dont worry its nothing"
but a funny way to say it
Mom: GO CLEAN YOUR ROOM
Me: mom dont fret your freckles ill do it later
Later: Mom: HAVE YOU CLEANED YOUR ROOM YET
Me: MOM I SAID DON'T FRET