Okay, so you know how you have your favorite toys and your friend has their favorite toys? Well, comic book characters are kind of like favorite toys for grown-ups. DC Comics is a company that makes comic books with lots of different characters like Batman, Superman, Wonder Woman, and many others.
Now, sometimes the characters from different comic books all go on adventures together. This is called a "crossover event". It's kind of like when you and your friend blend your favorite toys together and make up a new game.
DC Comics has had a lot of crossover events over the years, and they usually publish them in big comic books that have lots of pages. Sometimes the events are just one story, but other times they'll happen across different comic book series.
For example, one big crossover event was called "Crisis on Infinite Earths". This was a really special event that happened in the 80s. You know how sometimes there are different versions of the same character? Like, there's a cartoon Batman, a movie Batman, and a comic book Batman? Well, in the 80s there were SO many different versions of DC Comics characters. The "Crisis on Infinite Earths" story was about all of these different versions of the same characters coming together to fight a big bad guy who was trying to destroy the whole universe.
There have been lots of other crossover events over the years, like "Infinite Crisis" and "Final Crisis". Sometimes the events are really popular and lots of people buy them, and other times they're not as popular. But DC Comics keeps making them because it's fun to see our favorite comic book characters work together to fight the bad guys.