The birthplace of Muhammad, and thus the holiest city for Muslims. Muslims face in the direction of Mecca when they pray, and they are expected to go on a pilgrimage to Mecca at least once in their lives. Mecca is in present-day Saudi Arabia.
Note: Figuratively, a “mecca” is any place that attracts a great many people, especially for a particular reason: “Vail is a mecca for skiers.”