The Masjid al Haram is a mosque in the city of Mecca (or "Makkah"). It is considered by Muslims to be the holiest place on Earth and is the focal point of the hajj pilgrimage required of all able-bodied Muslims as one of the Five Pillars of Islam.
According to Islamic tradition, the mosque was built by Ibrahim (Abraham) with the help of his son Ishmael. They were ordered by Allah to build the mosque, and the Kaaba contained therein is supposed to be a model of the house of God in heaven. The Kaaba also contains the Black Stone which supposedly fell from heaven and turned black due to man's misdeeds. The story of Ishmael and his mother's search for water also takes place in the general vicinity of the mosque. In the story, Hagar runs between the hills of Safah and Marwah looking for water for her son, until God eventually reveals to her the Zamzam Well, from where water continues to flow to this day. The episode of Hagar's frantic search for water is emulated by Muslims as they run between the two hills whenever they visit Mecca.
The mosque itself has always been an important part of Arab culture. Even in pre-Islamic days it was a site for pilgrimages by pre-monotheistic Arabs, who filled the Kaaba with idols of the gods they worshipped. This episode is seen as a corruption of what the Kaaba was meant for, and the fact that they had strayed from the path of God explains why God sent Muhammad as a messenger to the people of Mecca. Muhammad's grandfather, Abu Muttalib, was a caretaker of the Kaaba, and Muhammad spent much time there in prayer and reflection.
After the Hijra, Muhammad was forced to withdraw to Medina. Upon his return to Mecca years later, he ritually smashed all the idols in the Kaaba and cleansed it. This began Islamic rule over Kaaba, and the building of a mosque around it. Initially, the mosque was much smaller than it is today but, over time, it has been gradually expanded. Under the Ottomans it grew to the roughly the size of the current courtyard. Great expansion work has taken place under the Saudi regime, along with modernization efforts. The mosque was much enlargened to handle the millions of visitors it receives during the hajj. It has also been outfitted with modern conveniences such as air conditioners and escalators. The outside of the current mosque has a splendid marble facade, and it has three stories, each which can hold thousands of worshippers.