Back in 2004, in WordPress 1.2 plugin was introduced in this platform. Since then, plugin is the most important part of WordPress system. WordPress has given us enough flexibility to extend the functionality by plugin.
We all are already used to with plugin. Whenever we need a plugin we can keep deactivated that plugin, or activate it when we need. But, sometimes we need something important and we always want to keep that functionality active in our WordPress site. Here comes mu-plugins. “mu-plugins” means Must Use Plugins – the plugins is always activated and there is no way to deactivate that plugin.