This was an annexe added to temples in the later historical periods where the annual rites associated with the birth of the god-king were performed. Scenes of the divine marriage and king's birth occur around the interior walls. Egyptologists refer to these annexes as 'mammisi'—a term derived from the Coptic 'place of birth'.
Biographical Dictionary of Ancient Egypt by Rosalie and Antony E. David
Ancient Egypt. A Reference Guide. EdwART. 2011.