The word tafsīr is the form taf'eel from the word al-fasr which means to explain (bayan). You say I explained the thing (without pronouncing the tashdeed), 'afsuruhu fasran', and I explained it (with the tashdeed); 'ufassiruhu tafsīran' when you explained it. The difference between tafsīr and ta'weel is that tafsīr is the explanation of what is intended by the wording and ta'weel is the explanation of what is intended by the meaning. The word tafsīr is chosen when applying to the explanation of the verses of the Qur’ān. The Qur’ān has been revealed in the Arabic language so its expressions are Arabic, including the expressions which are of a foreign origin like istabraq (brocade). It is has been Arabized according to the principles of Arabic and it became part of the Arabic expressions. The style of the Qur’ān is the well-known way of Arabs in their speech. He  said:

The Arabs used to recite it, comprehend the strength of its eloquence and understand its meanings. But not all of the Qur’ān can be approached by all Arabs who would understand it generally and specifically just by merely listening to it because by revealing the Qur’ān in the language of the Arabs does not mean that all Arabs will understand its words and phrases. Since not every book written in a certain language can be understood by the people of that language. The understanding of a book does not require language only but also a level of intellect in understanding and comprehension which agrees with the level and elevation of the book. The reality of the Arabs when the Qur’ān was being revealed was that not all of them were able to understand the Qur’ān generally and specifically. Rather, they used to differ in their understanding according to their intellectual elevation. Because of that reason the Sahabah's ability to explain and understand the Qur’ān was at variance due to the disparity of their understanding of the Arabic language and also due to the disparity in their intelligence and comprehension. The Qur’ānic expressions themselves and their meanings were not all understood by all Arabs. Anas ibn Malik narrated that a man asked 'Umar ibn al-Khattab about His  saying:

What is the abba? 'Umar replied:

It has also been narrated about 'Umar that he was on the minbar and he read:

Then he inquired about the meaning of takhawwuf? A man from Huzayl said: takhawwuf for us is the gradual decrease (tanaqqus).

Furthermore, there are many verses in the Qur’ān for whose understanding knowledge of linguistic expressions and styles is not sufficient. Rather, they require information about certain expressions because these expressions point to specific meanings such as in His  saying:

And other such verses which point to well known meanings, there are other verses whose understanding requires knowledge of the causes of revelation.

There are verses in the Qur’ān which are muhkam (explicit) and clear in meaning. They are the verses especially the Makkan verses which pertain to the fundamentals of the deen in terms of the ‘aqīdah and the verses which pertain to the fundamentals of the rules which are the Madinan verses, especially those related to transactions (mu'amalat), punishments ('uqubat) and testimonies (bayyinat). There are mutashabihat (ambiguous) verses as well in the Qur’ān which are ambiguous in meaning for many people, especially the verses that are open to a number of meanings or necessitates leaving the apparent meaning for another meaning because of the contradiction with the creed which should be free of anthropomorphic elements.

Even though the Sahabah were the most competent in understanding the Qur’ān because they were the most knowledgeable in the Arabic language and because they witnessed the circumstances and events around which the Qur’ān was revealed. However, they differed in their understanding and they differed in their ability to explain (tafsīr) the Qur’ān due to the disparity in the level of their familiarity with the Arabic language and due to the disparity in their closeness to the Messenger . The most famous Mufassirun from the Sahabah were 'Ali ibn Abi Talib, 'Abd Allah ibn 'Abbas, 'Abd Allah ibn Mas'ud and Ubay ibn Ka'b, they are the four who nurtured the most amount of exegetical material to the different Muslim cities. What enabled them to have such a deep knowledge of tafsīr was their strong understanding of the Arabic language, their grasp of its rhetorical form and styles, their mixing with the Prophet  and being close to him which enabled them to know the events for which verses of the Qur’ān were revealed and their faculty of intellect and intelligence, a faculty which enabled them to link meanings together in the best manner and come out with correct results. That is why they did not refrain from making Ijtihād in understanding the Qur’ān according to what the mind demanded. Rather, they made Ijtihād in tafsīr and spoke about it within their own opinions (Ijtihāds) and they made decisions based on what they had arrived at by their understanding and Ijtihād. Therefore, the tafsīrs of those people are considered as one of the highest forms of tafsīr. But many have lied about them and sayings have been interpolated in their tafsīr which they did not say, that is why you will find many fabrications in their tafsīr. What has been authenticated by trustworthy narrators is the strongest of tafsīrs. As for everything else from the fabricated reports it is not permitted to take them since it has not been proven that they said them. However, the caution of taking fabricated tafsīrs of those four does not mean it is a caution for reading their tafsīrs. Rather, it is a caution against taking them and practising them given the consideration that these are fabrications. As for reading them and determining a correct understanding by the language, Sharī’ah and intellect with what has been mentioned in them, this is useful because there are valuable explanations (tafsīrs) in these fabricated reports in terms of understanding even though their chains of transmission are weak in terms of their ascription to the Sahabah.