A registry is where images are stored. The most common of these is Docker Hub. This is normally where custom Dockerfiles pull images down from to build custom images. Docker Hub allows you to search for both official and unofficial images to be used in your projects, extended into custom images, or studied to see how the whale sausage is made.
NOTE: The trainers do not condone the hunting of whales for the making of whale sausage. Whales are beautiful, endangered creatures.
It is possible to add custom registries or even private registries to your Docker setup, but that is outside the scope of this training. Please consult the Docker docs