O Jibrīl, I have been sent to an illiterate nation among whom there are the elderly woman, the old man, the young boy, the young girl, and the man who have never read a book

The Messenger of Allah (may Allah's peace and blessings be upon him) met Jibrīl and told him that he had been sent to an illiterate nation who could not read.

By this, the Prophet (may Allah's peace and blessings be upon him) meant that if he instructed people to recite the Qur'an according to one Harf (mode of recitation/style of Arabic language), they could not do it, because some of them pronounced words differently, others were too old to learn, others were too young to read, and others had never read anything that was written before.

So Jibrīl told the Prophet (may Allah's peace and blessings be upon him) that the Qur'an was revealed in seven Ahruf (plural of Harf), so each person could read it in a way that was easy for them. Scholars have differed greatly on what the seven Ahruf meant. It seems (and Allah knows best) that they refer to the different styles of pronouncing the Arabic language.

The Qur'an was revealed according to those styles to make things easy at the beginning of Islam, as the Arabs were dispersed then, in disagreement, and every tribe had their own dialect. Some tribes would have had a linguistic style that other tribes did not. However, when Islam brought them together, and they were united and harmonious under Islam, each tribe knew the linguistic style of the other tribes.

‘Uthmān ibn ‘Affān (may Allah be pleased with him) got the people to recite in one Harf and burned the other copies of the Qur’an that were written in other Ahruf, so there would be no differences in the recitation of the Qur'an.