Must-use plugins (a.k.a. mu-plugins) are plugins installed in a special directory inside the content folder and which are automatically enabled on all sites in the installation. Must-use plugins do not show in the default list of plugins on the Plugins page of wp-admin (although they do appear in a special Must-Use section) and cannot be disabled except by removing the plugin file from the must-use directory, which is found in wp-content/mu-plugins by default. For web hosts, mu-plugins are commonly used to add support for host-specific features, especially those where their absence could break the site.