The Dockerfile
A Dockerfile is how you define your Docker container. You can write a Dockerfile that builds an application (Python, Node, .NET, etc) in the container's environment, where you can provide build arguments & environment variables, and run the resulting program within the container. This eliminates the need to install & maintain build & runtime tools on your host.
The Docker hub is a repository of Dockerfiles others developers have created. You can download their containers and execute them on your own machine with docker run, or with a Docker Compose compose.yml file. This documentation will go over both, as well as building & running your own containers.