From Halopedia, the Halo wiki
Mu was a continent of Arcadia, a human Outer Colony in the Procyon system. A number of provincial regions existed within the continent. After the rest of the planet was abandoned due to the Battle of Arcadia, a small group of settlers established the rural community of Abaskun. The settlers supplied the UNSC with much needed foodstuffs until the colony was attacked again by the Covenant in 2549 and the continent was glassed.
There was seven existing provincial regions on the continent.
Mu is most likely named after a mythical lost continent of Earth legend, which has been variously placed in the Atlantic or Pacific oceans.