A plug is something that blocks a hole, particularly one that water normally flows through. For some reason, it also means to promote something. If you plug your new book, you sneak it into every conversation.
You can buy a plug at a hardware store — it might be made of rubber, so it fits snugly in your kitchen sink drain. There are also accidental plugs that block the flow of water through a pipe, and plugs that fit securely in wall sockets. These plugs form a connection between an electrical source and an appliance, rather than stemming the flow. And to plug means to stop up or block, or alternately, to try hard to promote something.