Praise be to Allah.
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.”
To support that, Abu Hurayrah narrated about Umm Al-Qur’an that the Prophet said, ‘It is Umm Al-Qur’an, the seven repeated (verses) and the Glorious Qur’an.’ – Imam Ahmad recorded.
In another report also explain why Suratu Al-Fathiah is called Umm Al-Kitab. Abu Ja`far, Muhammad bin Jarir At-Tabari recorded Abu Hurayrah saying that the Messenger of Allah said about Al-Fatihah, ‘It is Umm Al-Qur’an, Al-Fatihah of the Book (the Opener of the Qur’an) and the seven repeated (verses).’
In conclusion, with all the narrations about you should be able to understand the Reason Surat Al-Fatihah is called Umm Al-Kitab. This could be because it contains the meaning of the entire Quran, the Quran start with it or the reason could be because prayer starts with Suratu Al-Fathiah.