Docker is an open source software platform that supports containers, making application deployment portable and self-contained. It is used to package the entire application (including SQL and NoSQL databases, Sonar configuration, and so on) and its dependencies together as a single container image, to deploy and test on any environment.

JHipster provides support for Docker out of the box for monolithic and microservice-based applications. Docker was developed for Linux, but has separate versions for macOS and Windows. JHipster only creates a Dockerfile at the time of application generation. The Dockerfile holds the set of instructions used by Docker containers to build a Docker image. 

JHipster also supports pulling Docker images from the Docker Hub. This is an online registry that's used to publish public and private Docker images. This greatly helps in using third-party tools without local installation, as Docker images can be pulled and run on a local container.