Pope says here that "true wit," by which he means a wise, intelligent and incisive statement or turn of phrase, reflects a truth that already exists in nature but dresses it up or highlights it for people. True wit is not saying anything new or inventing anything new, but unearthing what already exists and making it clearer to people. It brings to clearer consciousness "what oft was Thought," in other words what was already hovering in a cloudy way in people's minds.
However, before a person put the right words around a preexisting truth, that truth was somewhat inchoate (chaotic) or half-formed in people's minds. So people thought it but couldn't express it well: it was "ne'er [never] so well Exprest [expressed]." But once the right words describe the truth in question, we instantly know it to be true: that is the meaning of "Whose truth convinced at Sight we find." (We are convinced of the truth at the sight of the words that describe it well.) These words give us "back the Image of our mind." In other words, the words express what we already think, even if we never had words for the thought.
This is an important verse because it expresses a key Enlightenment belief in a nutshell. The Enlightenment thinkers believed objective truth could be found and that language was a vehicle to make that truth clear. The right language, the perfect words, what never was so well expressed before, became a clear windowpane revealing truth to people. Poor, murky language, on the other hand, was like a foggy or dirty windowpane: it made it difficult to see the truth. Pope here celebrates the person who uses language in a way that illuminates truth.