Docker 101 is a document that introduces Docker as a lightweight virtualization technology that allows developers to build, ship, and run applications inside containers that share resources from the host machine's operating system kernel. It explains that Docker uses Linux namespaces and cgroups to isolate containers and shares resources efficiently. It also provides an overview of Docker commands like build, run, and pull for creating Dockerfiles and images, running containers, and pulling prebuilt images from Docker Hub.