docker images outputs all images currently available.

By specifying -f "dangling=true" we restrict the list to "dangling" images (i.e. untagged).

By specifying the -q option we make the output quiet, so it prints only the image hashes.

We pass the output of $(...) to docker rmi, which removes the images with the corresponding hashes.

If there are no untagged images, than the docker rmi command will fail, since there is nothing to remove.