This is the magnificent mosaic of the Virgin Maria and the Child on the semi-dome of the absis.
This is believed to be the first panel containing human figures made after the Iconoclastic period. There is evidence dating it somewhere before the year 867.
Many of the Orthodox images, fortunately, were not destroyed by the Otoman rulers when Hagia Sophia was converted to a mosque. Because in Islam tradition, human figures are not acceptable in places of worship, these treasures were covered with Muslim content.
It was very fortunate that the Otoman rulers were lovers of beauty and art.