Google Cardboard is a special kind of viewer that you can put your phone in to make it look like you are in a 3D world.
It's like a pretend pair of glasses that makes cool things appear in front of your face.
You download special apps that work with Google Cardboard, and when you look through the viewer, you can see all sorts of things in 3D, like animals, outer space, and even roller coasters!
It's a fun and simple way to explore virtual reality and experience things that you might not be able to in real life.