Let's pretend you have a big toy box with lots of different toys inside. There are little tags on each toy that tell you things about it, like what color it is, how it works, and how to take care of it.
Well, computers have something similar called PCI configuration space. It's like a big toy box with lots of different gadgets inside (like video cards, sound cards, and network cards). And just like the tags on your toys, there are little tags on each gadget that tell the computer important information about it.
This information could be things like what type of gadget it is, how fast it can go, and how many things it can connect to. The computer needs to know this information so it can use the gadget properly and make sure everything works together smoothly.
So, the PCI configuration space is like the toy box for the computer's gadgets, and the little tags on each gadget are like the information that the computer needs to make everything work together.