A shared universe is like when you and your friends all play with the same toys and make up stories together. Each person has their own toys and characters, but they all exist in the same world and can interact with each other.
For example, if you have some toy cars, your friend has some toy animals, and another friend has some toy robots, you can all play together and imagine that the cars are driving around the animal kingdom with the robots helping them out.
In a shared universe for movies, TV shows, or comic books, different characters and stories can exist in the same universe and sometimes even cross over and interact with each other. So, like in the toy example, there may be superhero movies, sci-fi movies, and fantasy movies that all take place in the same universe and sometimes characters from one movie make an appearance in another movie.
Think of it like a big playground where different stories and characters can play together and have fun in one big world.