Complex sentence: A mihrab is semicircular niche in the wall of a mosque that indicates the "qibla"; that is, the direction of the Kaaba in Mecca and hence the direction that Muslims should face when praying.
Split into simpler sentences: A mihrab is a semicircular niche in the wall of a mosque that indicates the "qibla". That is the direction of the Kaaba in Mecca and hence the direction that Muslims should face when praying.