Court dress is the fancy clothes that people wear when they go to important places like a fancy party or a ceremony. There are different kinds of court dress for different types of events.
For example, if a person goes to a royal wedding, they might wear a long fancy dress or a suit that is very fancy with shiny buttons and a bow tie. Sometimes women wear hats that match their dress and men wear shiny shoes that look like they've been polished for hours.
Even judges in a court room wear special court dress. They wear a long black robe that is very serious and important looking.
People wear court dress because it shows that they are being very respectful and formal for the event they are attending. It's like putting on your best clothes when you go to a wedding, only court dress is even fancier than your best clothes.