What is the difference between base URL and mirrorlist in Yum?
The base URL is just the standard location of the repository that has all of the software for the repo whereas the mirrorlist is just other locations that it will search in case it can't can't connect to the base.
Think of it as the site where you download ISOs for a Linux distro such as Ubuntu or Fedora. There's the main site and then other mirrors that will have it case you can't get it from the main site.
The difference is that the mirrorlist is there to increase availability in case the base can't be reached.