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
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.