Okay kiddo, you know how there are different superhero movies with different characters like Superman, Batman, and Wonder Woman? Well, the DC Extended Universe (DCEU) is a bunch of those movies all connected together, like a big puzzle.
This means that the characters from one movie might show up in another movie, or they might talk about something that happened in a different movie. It's like they all live in the same world and interact with each other.
The DCEU started with a movie called "Man of Steel" which introduced us to Superman. After that, we got movies like "Batman v Superman: Dawn of Justice" where Batman and Superman have to fight each other, and "Wonder Woman" where we learn about her backstory and how she became a superhero.
There are also other characters like Aquaman and The Flash who have their own movies in the DCEU. And sometimes they all team up together, like in "Justice League" to fight against really big bad guys.
So, basically, the DC Extended Universe is a collection of superhero movies that all fit together and tell a bigger story. It's like a big family of superheroes that live in the same universe and help each other out. Pretty cool, right?