Debutante is a fancy word for a girl (usually from a rich family) who is celebrating her coming-of-age. This means she is becoming an adult and is ready to join high society. It's kind of like a big party where people wear fancy clothes, eat fancy food, and dance with each other.
The debutante is the star of the show and gets to wear a beautiful ball gown and sit on a throne-like chair to greet all the guests. It's a big deal because she is now considered grown-up and ready to be introduced to the world as a lady.
This tradition started a long time ago in Europe and is still popular in some parts of the world, especially in the United States. It's not for everyone and can be quite expensive, but some families like to celebrate this milestone with a big event.