The Reason it is called Umm Al-Kitab

In the beginning of the Book of Tafsir, in his Sahih, Al-Bukhari said; "It is called Umm Al-Kitab, because the Qur'an starts with it and because the prayer is started by reciting it.'' It was also said that it is called Umm Al-Kitab, because it contains the meanings of the entire Qur'an. Ibn Jarir said, "The Arabs call every comprehensive matter that contains several specific areas an Umm. For instance, they call the skin that surrounds the brain, Umm Ar-Ra's. They also call the flag that gathers the ranks of the army an Umm.'' He also said, "Makkah was called Umm Al-Qura, (the Mother of the Villages) because it is the grandest and the leader of all villages. It was also said that the earth was made starting from Makkah.''
Further, Imam Ahmad recorded that Abu Hurayrah narrated about Umm Al-Qur'an that the Prophet said,

«هِيَ أُمُّ الْقُرْآنِ وَهِيَ السَّبْعُ الْمَثَانِي وَهِيَ الْقُرْآنُ الْعَظِيمُ»

(It is Umm Al-Qur'an, the seven repeated (verses) and the Glorious Qur'an.)
Also, Abu Ja`far, Muhammad bin Jarir At-Tabari recorded Abu Hurayrah saying that the Messenger of Allah said about Al-Fatihah,

«هِيَ أُمُّ الْقُرْآنِ وَهِيَ فَاتِحَةُ الْكِتَابِ وَهِيَ السَّبْعُ الْمَثَانِي»

(It is Umm Al-Qur'an, Al-Fatihah of the Book (the Opener of the Qur'an) and the seven repeated (verses).)