Docker images consist of multiple layers. Dangling images are layers that have no relationship to any tagged images. They no longer serve a purpose and consume disk space. They can be located by adding the filter flag, -f with a value of dangling=true to the docker images command. When you're sure you want to delete them, you can add the -q flag, then pass their ID to docker rmi:

To list dangling images:

docker images -f dangling=true

To remove dangling images:

docker rmi $(docker images -f dangling=true -q)