The Temple Mount is sacred to Muslims, Christians, and Jews. It considered the third holiest site in Islam, after Mecca and Medina: the Prophet Mohammad is believed to have traveled there in a dream, and Muslims believe it is the site where it is believed he ascended to heaven; the al-Aqsa mosque was built to commemorate this journey.
The Temple Mount is also the site where Jews believe that God revealed the Talmud to them, and where a number of ancient temples were built by Jewish Kings (such as King Solomon), so it is considered equally important to Jews. The Temple Mount, where the al-Aqsa mosque is located, is part of the Old City compound, which is under Israeli control; however, the site itself is guarded by Muslims, and there is an understanding that, while anyone is free to visit the site (as I have done), only Muslims may pray there. 