Muslims universally pray in the direction of Mecca as a symbol of religious unity and because this Saudi Arabian city is home to the Masjid al-Haram mosque or, more specifically, the Ka'bah inside its walls. The Ka'bah, a large black cubical building, is believed to have been built by Ibrahim (Abraham) and his son Ismail (Ishmael), and is considered to be the holiest site in the Islamic world.
The Ka'bah is also said to comprise, as its eastern cornerstone, the Black Stone. Thought to have been given to Adam at the time of his Fall with Eve from Eden, it is believed to have blackened from its original white hue with the sins of all those pilgrims who have come into contact with it.
Sometimes people struggle to ascertain the direction of Mecca, as was the case in Indonesia, which officially altered its citizens' praying direction from the west (Africa) to the northwest (Mecca) in 2010.