Umm-Ul-Momineen or Ummahat-ul-Muʾminin, is the term used for Prophet Muhammad'sﷺ wives. And they are called the Mothers of the Believers رضي الله عنهم, as a way of honouring them and giving them a noble position.
Muslims refer to them as Mothers of the Believers. Muslims use the term prominently before or after referring to them as a sign of respect. Allah prohibited them to marry anyone after the death of the Prophet Muhammadﷺ. They were the best Muslim women in piety and righteousness.
They supported the Prophet Muhammad ﷺ, during his lifetime and they shouldered the responsibility of calling people to Islam and spreading the message, and teaching the Muslim women the rulings of Islam.
The Prophet is more worthy of the believers than themselves, and his wives are [in the position of their mothers. Qur’an.Surah Ahzab 33:6
Wives of the Prophetﷺ! If any of you commits an obvious act of indecency she will receive double the punishment. That is an easy matter for Allah. But those of you who are obedient to Allah سبحانه و تعالىand His Messengerﷺand act rightly will be given their reward twice over; and We have prepared generous provision for them. Qu’ran.Surah Ahzab: 30-31
The Prophet Muhammad ﷺalso told his companions how they should treat their own wives:
The Mothers of the Believers رضي الله عنه, were the best Muslim women in piety and righteousness. They supported the Prophetﷺ, during his lifetime and they shouldered the responsibility of calling people to Islam and spreading the message, and teaching the Muslim women the rulings of Islam. 
The first of the wives of the Prophet Muhammad ﷺwas Khadija رضي الله عنه. She was also one of the very first Muslims. When the Prophet ﷺreceived his first revelation he immediately told her of it. Khadija رضي الله عنه is known for her intelligence, perceptiveness, foresight and wisdom, and she immediately believed, offering the Prophetﷺenormous support in the days that followed and expending great physical and spiritual effort to help spread the morality of the Qur'an.
Other wives of the Prophetﷺ, such as Sauda, Aisha, Hafsah, Umm Habiba, Umm Salama, Safia, Moyomuna, Zainab bint Jahsh, Juwairya (Radhiallah Anhum) are also mentioned for their self-sacrifice, patience, and loyalty to the Prophet ﷺ, and were also excellent role models for the faithful.