What is meant by the saying of the Prophet, peace and blessings be upon him: "Whoever speaks about the Qur'an with his own opinion has erred, even if he is correct"? And does this mean not interpreting the Qur'an and merely sufficing with what the Companions said?

The intent of the Hadith (definition) is to warn against interpreting the Quran without knowledge. Whoever interprets the Quran based on his own opinion has undertaken something of which he has no knowledge and has erred, even if he arrived at the correct meaning, because he did not approach the matter through its proper channels. The fundamental principle in interpreting the Quran is to interpret it first with the Quran, then with the Prophetic Hadith, then with the sayings of the Companions, and then with the sayings of the Successors. Whoever deviates from these principles is mistaken, even if he reaches the correct conclusion.