Decorum can be defined as behavior that is in keeping with good taste and propriety. It was a very important concept in neo-classical poetry. The general consensus in the 18th century was that poetry should be written according to certain rules and standards. As such, there was a prevailing distrust of anything unusual, particular, or in any way innovative. Poetry should speak to common human experience and not simply be the eccentric outpourings of a tortured individual. Emotions certainly had their place in poetry, but only those emotions that are widely shared, understood and recognized; and even then they should be expressed with appropriate decorum and restraint. Otherwise, poets were little better than religious cranks and fanatics who claimed to have a unique inner light, a divine inspiration that revealed to them the absolute truth.

For Pope, as for many in the 18th century, the proper study of mankind is man. And poetry should concern itself with uncovering universal truths that speak to man's reason. Inevitably, this meant that poetry needed to be written according to an objective standard, one that could be understood and appreciated by anyone exercising their reasoning faculties. Hence the need for commonly-accepted rules of taste and propriety against which poems, as with all works of art, should be judged.

Two quotes from the Essay can probably provide us with the best answer to this question.

First of all, unlike not only the later, Romantic period but to an extent the preceding periods of modern literary history, the neoclassical artists did not prize “originality” as we understand it, or see it as a component of decorum. Therefore:

True wit is Nature to advantage dress’d,
What oft was thought but ne’er so well express’d.

What does this mean in practical terms? Pope and others of the period did not express personal sentiments in their verse apart from a few notable exceptions, nor were they interested in overturning existing ways of thinking. At the same time their intention was to express accepted modes of thought (and these are what “Nature” is composed of) in an elegant and polished format in which the reader would admire the beauty of the wording as much as the fact of its expression of timeless truths. This brings us to the quote which, more than anything, sums up not only Pope’s aesthetic ideal but that of what we call “classicism” (not merely “neoclassicism”) in poetry or in any of the arts:

’Tis more to guide than spur the Muse’s steed,
Restrain his fury, than provoke his speed.
The winged courser, like a gen’rous horse,
Shows most true mettle when you check his course.

In understanding this we have to be aware that Pope and others of his period did not literally believe they were putting a check on their emotions or withholding the expression of inner sentiments. They saw no disconnect between universal belief and personal thought or emotion. It was, rather, that true expression lay in the kind of perfectly sculpted verse (invariably in heroic couplets in Pope’s oeuvre, with rare exceptions) in which wayward or transient thoughts were subordinated to a kind of uniformly approved set of ideas, and the techniques of putting them into words. This was true decorum for Pope.