When you do commands like
docker rm $(docker ps -aq), how do you call the
$( ) part?
$() creates a sub-shell to run the command in the brackets, and captures the output such that you can feed that output to another command. It is a very powerful technique in bash and other similar shells.
docker rm $(docker ps -aq) - will be interpreted like so
docker ps -aq
docker rmusing the output of #1 as the argument - i.e. the container(s) to remove