As-Salaam alaikum,
In discussing the term ummi, one has in mind the sobriquet of the Prophet Muhammad, Sallallahu alaihi Wasallam, the "unletterd Prophet" (Qur'an-7:158). This means that the Prophet's knowledge came only from Allah, not from reflection and consideration. This is further supported by other verses of the Qur'an such as that his knowledge was "from Allah" or "Allah-given" (ladunni) : "We had taught (Khidr) knowledge from US" (18:65)
In his book, "Futuhat al-Makkiyya", Sidi Muhiyiddin Ibn Arabi, commenting on this observed that :--
"For us, being" unletterd "does not contradict memorizing the Qur'an or the prophetic Hadith. In our view, the person is unletterd who does not employ his reflective consideration and his rational judgment or bring out the meaning and mysteries which the Qur'an embraces. He does not use rational proofs to attain to the knowledge of divine things. And he does not employ the juridical proofs, analogies, and assigning of causes that occupy the legal authorities in order to grasp the rulings of the Law.
When the heart is free from reflective consideration, then, according to both law and reason, it is "unlettered" and receptive toward the Divine opening in the most perfect manner and without delay. It is provided with Allah-given knowledge in all things to an extent unknown except to a Prophet or one of His friends whom He has given of it through tasting.
Through this knowledge the degree and plane of faith are perfected. Through it the one who receives it becomes aware of the correctness and the mistakes of reflective powers, and in what respect soundness and disorder are attributed to them. All of this comes from Allah".