"Legalize it" is the catch phrase often used by proponents of Marijuana de-criminalization. It is also the title and cheif lyrical component of a song by reggae artist Peter Tosh.
The slogan is also something of a gag within popular culture. For instance, in "The Simpsons" episode where Lisa learns of her future as president of the United States, and Bart has to bail her out of a crushing national debt, his one requested favor in return is to "legalize it."
"Legalize what?" Lisa responds.
"It," says Bart.
She gets it.