The Prophet received the Quran by revelation, but he did not know the meaning of the Quran as a whole.

He was asked about many ayat, and he could not answer, but only to say: "I don't know; I heard it from the angel as such; God is the All-Knowing."

Moreover, Muslims (and others of course) do not know the explanation of many ayat in the Quran.

That is because the Quran was revealed in two categories: plain and ambiguous (or puzzles.)

The puzzles will be solved in the future by the Mahdi, who will receive the inspiration so as to interpret the ambiguous ayat of the Quran, and will be his miracle, because Muslims have not till now known the meaning and implication of the puzzling ayat which only God knows.

This will also be the miracle of the Quran, which will be proved authentic and it is truly the word of God.

That is because the Prophet himself who brought it, did not know it, but only he conveyed it to people honestly as he heard and received it from the angel.