What does the DOCKER_HOST variable do?

To solve $docker_host you can go through the below-mentioned solution:

In this case, Client is the docker command in OSX

Host is basically the boot2docker virtual machine.

Daemon is basically a service running in the background inside boot2docker.

When you type in the docker commands they work because the terminal window that comes up already has the DOKCER_HOST set . You will be required to set this variable in some other windows.

another thing you can do is use the below command:

$ export DOCKER_HOST=tcp://192.168.59.103:2375
This should set the DOCKER_HOST variable,but it would be better to try this:
$ $(boot2docker shellinit)
$ docker run hello-world
Hello from Docker. [...]
Trying to run boot2dockerr shellinit gives back a group of bash commands that set the environment variable :
export DOCKER_HOST=tcp://192.168.59.103:2376
export DOCKER_CERT_PATH=/Users/ddavison/.boot2docker/certs/boot2docker-vm
export DOCKER_TLS_VERIFY=1
Just run $(boot2docker shellinit) it will generate all those commands
and then just run those commands .

Hope this helps