The lines to which you are referring in Alexander Pope's "An Essay on Criticism" are:
297 With gold and jewels cover ev'ry part,
298 And hide with Ornaments their want of art.
299 True wit is nature to advantage dress'd,
300 What oft' was thought, but ne'er so well express'd;
These lines occur in a section describing types of bad critic who to "Conceit alone their taste confine". Conceit refers to a very elaborate type of extended metaphor that draws parallels between the intellectual and spiritual worlds and the physical world, often using very technical scientific or philosophical vocabulary, which was favored by the Metaphysical poets. Pope objects to the use of conceits, seeing them as unpoetic. In this passage, he describes conceits as being like ornaments which mask a basic lack of good substance, rather as a chef might use lots of spices to cover up poor quality ingredients.
In the second of the two couplets, Pope suggests that real wisdom for a poet, as opposed to superficial cleverness, consists of shaping universal truths in pleasing form. His view is deeply religious, in that he sees nature as something created by God and understood instinctively by great ancient poets such as Homer. The modern poet, in Pope's opinion, should modestly try to understand these eternal truths rather than departing from them and valuing his own ego and originality instead.