Plug has several common meanings:

1. A device that connects an electrical appliance to a power source. The plug on your lamp has metal prongs that fit into a wall outlet, allowing electricity to flow and power the light. Without the plug properly inserted, the appliance won't work. Different countries use different plug shapes, which is why travelers often need adapters.
2. Something that blocks or fills a hole. A bathtub plug stops water from draining out. A cork is a plug for a bottle. When a hole in a boat starts leaking, someone might urgently search for something to plug the leak. Your finger can even serve as a temporary plug if you stick it in a small hole.
3. To promote or advertise something enthusiastically. When an author appears on a talk show to plug her new book, she's trying to get people excited about reading it. A friend might plug their favorite restaurant by telling everyone how delicious the food is. This kind of plugging happens when someone really believes in something and wants others to know about it.
4. To keep working steadily at something difficult. When you plug away at a challenging math problem or plug along on a long-term project, you're persevering even when progress feels slow.