When people in the United Kingdom go to fancy events that involve a lot of important people like judges or the queen, they wear clothes that are different from their everyday clothes. These special clothes are called "court uniform and dress."
Court uniform is what men wear. It can be black or grey and usually has a tail (kind of like a coat with a long back). There are also different decorations on the uniforms depending on what job the person has. For example, judges wear special uniforms with white collars and cuffs while people in the army might have medals or badges on their uniforms.
Women wear a special dress called a "court dress." These dresses are usually long and have many layers of material. They also wear fancy hats and gloves to match their outfits.
The reason people wear court uniform and dress is to show that they are important and have an important job. It's a way of showing respect for the event and the people there. When you're five years old, you might wear a pretty dress or a nice suit to a party or a wedding. So, just like how you wear your fancy clothes to special events, people in the UK wear court uniform and dress to important occasions.