A plugin that is uploaded to the “mu-plugins” folder is always activated by default and can not be accidentally deactivated by the user. It was originally created for multi-site blogs (as “multi-user” plugins that were active on all sites of a multi-site) but can now also be used for single sites.
- can not be deactivated
- are always executed
- can not use activation hooks
- loaded before all other plugins (in alphabetical order)
Visit the documentation for more.