"Ummi" in Qur'an 7:157 is generally taken to mean "unlettered". That is, the Prophet (S) did not read and write, and did not receive schooling. Therefore, the fact that he was able to provide a complex book with an eloquent style is further proof that the Qur'an was a divine miracle, since he did not have the education necessary to write a complex and eloquent book, or the ability to read previous scriptures and then write responses to them.
Sunnis tend to hold that the Prophet (S) was illiterate. Shi'is tend to hold that the Prophet (S) had the intrinsic knowledge of reading and writing, as part of his perfection and innate access to knowledge through Allah, but that he chose not to read or write. In any case, there is no evidence that the Prophet (S) ever read or wrote everything personally; instead, he had scribes take dictation for him (such as letters to others, or the Qur'an).
There are some mystical views which hold that the adjective "ummi" might reflect a sort of childlike simplicity or innocence. Especially since, in Islam, children are seen as being born pure and sinless. For instance, we refer to children as "innocent" because they don't have the sense of trickery, cunning, deceit, greed, or desire for power and carnal indulgence that some adults develop throughout life, and which can cloud their spiritual faculties. So, a person who is a mature adult but who nonetheless has a pure and "innocent" soul would still have a sort of childlike purity even if they are spiritually, intellectually, and socially mature. However this is not with respect to the Prophet's formal knowledge but rather - if it is a correct interpretation - would simply allude to an inner purity that would make him a suitable receptacle to receive the divine word in its purest form, just as if I want to write a text on a chalkboard, I will have the best results on a pure and clean chalkboard rather than one with a lot of marks.
In any case, it would be an additional interpretation to the above. Certainly the Prophet (S) demonstrated a lot of knowledge and wisdom in his life and management of the community so obviously his knowledge was well beyond that of a child.
It has also been suggested that "ummi" may refer to the designation of Mecca as the "mother of cities" (umm al-qurra) and therefore associate the Prophet with Mecca.
Maybe all of the interpretations have some truth but the first one is the most common ascribed to the text.