Plug is a slang term often use to refer to drug dealers. The term derives from a plug which you can connect electronic products to obtain power to operate said electronic device. Plugging someone means that you've connected them to someone they can purchase drugs from.
In Kap G's U.O.E.N.O Remix song, he says "Yea I just blow that ganja, nah I don’t sell that ganja - Got them migos up in Mexico, I can plug ya in if I know ya." Kap G is basically saying he smokes weed but doesn't sell it, but he has connects in Mexico, if you wanna start slinging, let him know and he'll plug you.