Core OpenGL is like a big box of crayons that helps a computer draw pictures on the screen. Just like how you use different colors of crayons to make a picture look pretty, a computer uses different parts of Core OpenGL to draw pictures and make them look cool.
It's called "Core" because it's the most important and basic part of OpenGL. It's kind of like the crust of a pizza - without it, you don't really have a pizza.
Core OpenGL has a lot of rules that help make sure everyone can understand how to use it. These rules make it easier for people to make cool pictures and share them with others.
So, to sum it up, Core OpenGL is like a big box of special crayons that a computer can use to draw cool pictures on the screen, and it has really important rules that make everything easier.