In order to understand this passage, you have to begin where the passage begins. It is no good beginning to analyze or summarize a passage of poetry where the punctuation indicates a continuing thought. You must begin where the thought in interest starts. In this case, the beginning of this passage is line 68, which is also the start of this stanza. Poetic line-end punctuation is critical to attend to and the important punctuation that is a full stop (period) is at line 67. The same rationale applies when determining to end a passage: end at the closest period to the bit worrying you or of interest to you. This passage ends at line 91. The passage begins:
First follow Nature, and your Judgment frame
By her just Standard, which is still the same:
Unerring Nature, still divinely bright, 
One clear, unchang'd and Universal Light,
Life, Force, and Beauty, must to all impart,
At once the Source, and End, and Test of Art.
An encapsulation of Pope's point in this passage is that poetry, inspired by the Light of Nature, can be violated--but must not be violated--by "Show" and "Pomp" and by "Wit" that lacks "Judgement." Rather, poetry must follow the "RULES" of poetry that were "discover'd, not devis'd" [discovered in previous great poems] and that are turned to poetic method under "Laws" of restraint that are similar to the laws that restrain "Liberty."
To briefly summarize the passage, Pope starts with identifying and defining poetry while linking it with "the Source, and End, and Test of Art." Poetry is said to be Nature and the "Universal Light, / Life, Force, and Beauty" that flows to all through Nature. Next, Pope turns to identifying the antithesis of poetry, which is poetry that is pompous and showy. More about antithesis later, but first he sidesteps and identifies the poet as one in whom the "Spirit" of Nature resides, filling the "whole" person with vigorous inspiration that "guides" every poetic impulse and creation. Pope clarifies that while the inspiring Spirit itself is not seen, its "Effects" upon the poet and the poetry produced are seen:
With Spirits feeds, with Vigour fills the whole,
Each Motion guides, and ev'ry Nerve sustains;
It self unseen, but in th' Effects, remains.
Pope returns to defining antithetical qualities by describing an antithetical poet. This individual is inspired not by the "Fund" of "Universal Light," but by "Wit" that lacks "Judgement." In other words, a clever verbose person who has not got a bit of artistic taste nor of good sense; someone who is a bit of a bore. Pope then explains how, in Nature, wit and judgement are united and how the "Spirit," now called the "Muse," is called upon in the same manner that one gently rides a good horse, without the "spur" and without provoking it to unrestrained "Speed." This hearkens forward to the upcoming comparison poetry to Liberty restrained by rules of order:
The winged Courser, like a gen'rous Horse,
Shows most true Mettle when you check [restrain] his Course.
Pope closes with an emphasis on rules of "Nature Methodiz'd" and with the comparison to "Liberty" restrained by laws that are innate to liberty, as rules and application of method are innate to Nature ... which is equated with Art ... which is poetry (Pope has gone full circle with his argument):
Nature, like Liberty, is but restrain'd 
By the same Laws which first herself ordain'd.
Alexander Pope's Essay on Criticism (1711) is one of the most important statements about literature in the Augustan Age, also characterized by the term Neoclassicism because its supporters, including Pope, advocated a return to the type of literature created during the Greek classical period.
One of Pope's major arguments in the Essay is that writers, especially poets, should follow Nature, by which he meant the guiding principle of the universe--the highly-ordered, rational, reasonable mind of God:
First follow NATURE, and your Judgment frame/By her just STANDARD, which is still the same:/Unerring Nature,still divinely bright,/One clear, unchang'd and Universal Light. . . . (ll. 69-71)
Pope's goal in these and the following lines is to convince writers that if they follow Nature's unchanging guidelines their writing will always remain true to the guiding principles of the universe, which readers will understand because they, too, desire universal truth.
Following Nature, according to Pope, is the only way for writers to achieve the ultimate goal of good writing:
Life, Force, and Beauty, must to all impart,/At once the SOURCE, and END, and TEST of ART. (ll. 72-73)
Here, Pope makes explicit that the proper goal of any art--and here he would include both literature and visual arts--is to reflect the beauty and truth of Nature and Nature's God rather than to create art that deviates in any degree from Nature's "Standard." Pope is taking a very conservative stance here because he is essentially arguing that there is only one kind of acceptable art--that which reflects Nature--and any art that fails to accomplish these goals is not worth producing.
In lines 74-83, Pope furthers his initial argument by pointing out that proper art achieves its goals "without Show, and without Pomp," that is, by reflecting Nature and truth, art doesn't draw attention to itself as art. And he argues further that those writers who are especially gifted with wit ("to whom Heav'n in Wit has been profuse") are tempted to exercise their wit, which, in Pope's view, is artificial and therefore not a proper component of art:
For Wit and Judgment often are at strife . . . 'Tis more to guide than spur the Muse's Steed. . . . (ll. 82-84)
In other words, poets who are gifted with wit often use that wit rather than appropriate judgment, and the two attributes are often incompatible during the creative process. Instead, according to Pope, wit should be used sparingly and always be tempered by good judgment (and good judgment is guided by Nature).
The poet's goal, as Pope argues in the last four lines of this section, is to tame his wit because his writing is most true to nature when "you check his Course." In other words, wit has a role in the creative process but must be subject to Nature's principles and the poet's judgment.
In essence, one of the principal goals of writers in the Augustan Age or Neoclassical Period (the early 18thC to about mid century) is to model literature and art on the principals of Nature and what they often referred to as "Nature's God," and, equally important, to exercise moderation in all things, especially literature and the visual arts. The "Test of Art" is, then, how closely art reflects Nature.