cmd defines a command that should be run inside the container.
You can explicitly pass the cmd as argument to the docker cli.
docker run cmd # start a container and run a cmd
docker exec cmd # run a cmd against a running container
Example:
docker run -i -t ubuntu bash
You can add a cmd 1) in a dockerfile
docker inspect --format "{{range .Config.Cmd}}{{.}}{{end}}" containerOrImageName