a person/drug dealer who is known for selling the best stuff for the best prices and for selling almost everything you can name or a person who has anything you need and that will give it to you if they got it
Yeah Adam's the plug he let me borrow his car to visit my girlfriend!!
A person who's got your back with anything you need. The expression comes from how a plug connects electronics to power them, similarly to how someone who is a 'plug' has everything their friends need.
*Joe approaches his friends Bob and Tom*
Joe: Hey guys I bought you cookies since I know you're hungry.
Bob: Shit thanks bro.
Tom: Joe is once again the plug.