This is an excerpt from Alexander Pope's Essay on Criticism. The full quote is as follows:
“Whoever thinks a faultless piece to see,
Thinks what ne'er was, nor is, nor e'er shall be,
In every work regard the writer's end,
Since none can compass more than they intend;
And if the means be just, the conduct true,
Applause, in spite of trivial faults, is due.”
It means that there is no such thing as the "perfect" piece of literature. If you're looking for such, you're asking for the impossible--for what never has existed, does not exist, nor ever will exist. Instead, you should consider what the writer's goal was and judge him by that. (We're all mere humans and cannot do more than we strive to do.) And judging by that, if the technique and style is good and the plot and characters true to life, you owe the writer your admiration and applause, even though the text itself suffers from trivial faults. In short, as an epigram, this quote asks the reader to not expect perfection, but to judge the work based on how well the writer does what she set out to do.