Okay kiddo, so Chaldea is kind of like a big magical organization that exists in a video game called Fate/Grand Order. Imagine it like a big castle where lots of magical people work together to protect history and save the world from bad guys.
In the game, you play as a master who controls a team of heroes and heroines from different places and times in history. These heroes are called "Servants" and they help you travel through time to fix events that might mess up history.
Chaldea is like your home base, where you go back to between missions. It's where you plan your next move, upgrade your heroes' abilities and weapons, and talk to other characters in the game.
So basically, Chaldea is a magical place where lots of cool heroes hang out and help you save the world. And it's a big part of the story in Fate/Grand Order.