Universal character set characters are just like the different letters you use to write words, except instead of just the letters in the alphabet, there are a lot more characters that people can use from all different types of languages and symbols all over the world.
Imagine you have a big box of crayons, and each crayon is a different color. Some of the crayons might be red or blue, just like how some characters are letters that are familiar to you. But there might be some crayons that are a different color like green or yellow, and this is just like how some universal character set characters might seem strange to you because they come from a different language or culture.
The really cool thing about universal character set characters is that they allow people from all over the world to communicate with each other even if they don't speak the same language. It's like having a big box of crayons that everyone can use to draw pictures and communicate with each other in their own way.