What does minikube docker-env mean?

In the Kubernetes minikube tutorial there is this command to use Minikube Docker daemon :

$ eval $(minikube docker-env)

What exactly does this command do, that is, what exactly does minikube docker-env mean?

The command minikube docker-env returns a set of Bash environment variable exports to configure your local environment to re-use the Docker daemon inside the Minikube instance.

Passing this output through eval causes bash to evaluate these exports and put them into effect.

You can review the specific commands which will be executed in your shell by omitting the evaluation step and running minikube docker-env directly. However, this will not perform the configuration – the output needs to be evaluated for that.

This is a workflow optimization intended to improve your experience with building and running Docker images which you can run inside the minikube environment. It is not mandatory that you re-use minikube's Docker daemon to use minikube effectively, but doing so will significantly improve the speed of your code-build-test cycle.

In a normal workflow, you would have a separate Docker registry on your host machine to that in minikube, which necessitates the following process to build and run a Docker image inside minikube:

Build the Docker image on the host machine.
Re-tag the built image in your local machine's image registry with a remote registry or that of the minikube instance.
Push the image to the remote registry or minikube.
(If using a remote registry) Configure minikube with the appropriate permissions to pull images from the registry.
Set up your deployment in minikube to use the image.

By re-using the Docker registry inside Minikube, this becomes:

Build the Docker image using Minikube's Docker instance. This pushes the image to Minikube's Docker registry.
Set up your deployment in minikube to use the image.

More details of the purpose can be found in the minikube docs.