A Must-Use Plugin (MU) is an essential extension module that remains permanently active on a website and cannot be deactivated. It loads first, before all other plugins, when pages load. Unlike traditional plugins, the MU plugin is automatically activated without requiring any action through the administration interface, preventing accidental deactivation by users. These plugins are manually installed and activated in the wp-content directory of WordPress, and are specifically designed for multisite environments while remaining compatible with standard websites. The MU plugin is crucial for making global changes or adding new features to the entire site, acting as an indispensable extension for its proper functioning.