"Downtown" is slang in NYC and elsewhere for heroin, as opposed to "uptown," which is crack or cocaine. This distinction is usually only necessary when the dealer happens to sell both heroin and crack or cocaine.
(Dealer's cell phone rings)
Dealer: Yeah, who's this?
Buyer: It's _______.
Dealer: What do you need?
Buyer: I need 2 downtown and 3 uptown.
Dealer: Go to 20th between Broadway and Fifth in 10 minutes.