Direct3D is a special kind of computer helper that helps video games and other programs make 3D pictures on your screen.
Think of your screen like a flat piece of paper. Normally, you can only draw a picture on the paper that looks flat too. With Direct3D, you can make things look like they're jumping out of the paper, like a real object!
Imagine you want to see a picture of a birthday cake in 3D. You would need to tell Direct3D how to draw the cake in 3D, and then it would magically make it look like you could walk around the cake and see it from all angles!
Direct3D is like your art teacher. It helps you draw, but it also helps you understand how to make things look 3D in your picture.
So, whenever you play a game or use a program that has cool 3D graphics, thank Direct3D for making it possible!