mu stands for must use, not for multisite. These plugins are stored in wp-content/mu-plugins by default, but one can change that in wp-config with two constants: WPMU_PLUGIN_DIR and WPMU_PLUGIN_URL.
Mu-plugins are loaded in alphabetical order for all sites in a network or a single site installation on each request before the regular plugins.
The first action after all mu-plugins are loaded is 'muplugins_loaded'.
Links:
