A crown is a fancy, round ornament worn on the heads of kings and queens. To crown someone is to declare them a monarch or a champion. A crown is also a boring old top of something, like a tooth.
Historically, a crown represented the country ruled by the monarch who wore it. In some cases, the person wearing a crown wasn't the king, but the head of a church, as most modern-day Catholic popes have done during certain ceremonies. Other kinds of crowns are the top of a hat or the top part of your head, and also the visible part of a tooth — or the artificial replacement made by a dentist.