A circlet is like a special kind of crown that is smaller and lighter. It is usually made of precious metals like gold or silver, and is decorated with jewels, pearls or other shiny things. The reason it's smaller and lighter is because it is meant to be worn around the forehead or the head, rather than on top of it like a regular crown.
Have you ever seen a pretty headband with flowers or sparkles on it that someone wears in their hair? A circlet is kind of like that, but fancier and more expensive.
In medieval times, queens and princesses wore circlets to show that they were royal and important. Nowadays, people might wear them for special occasions like weddings or formal events. They are also sometimes worn in fantasy stories or in movies about royalty, like in "Game of Thrones."