This is the mihrab --- or apse --- that indicates direction in which to pray and which is supposed to indicate the direction of the sacred Kaaba in Mecca. You'll notice that I said that the mihrab is supposed to indicate the direction of Mecca. In fact, the direction indicated actually points south rather than east-southeast. While I won't go into it here, there's a story about this anomaly that you can pursue on your own if you are interested.