It goes well beyond “grammar”
Editing means paying close attention. And being informed. It means questioning every detail from the smallest to the largest.
It means when you see a reference to “Bedlam” in a profile as “a storied asylum of yesteryear,” with the name glossed as meaning “chaos,” you fix it. That’s “the famous asylum,” and maybe you keep “storied,” you almost certainly ditch “yesteryear” (unless the writer is very good, she’s likely gone too far out on a limb with that one and you’ve got to reel her back). Whether you say simply “of long ago” or locate it more exactly depends on how much detail the reference will support before becoming overburdened, and you do fix that misleading gloss of the word. “Bedlam” is now synonymous with uproar, confusion, chaos. It didn’t start out that way. It acquired the meaning over time, by association with the asylum.
And if you didn’t already know that, you looked it up when you came across the word and the explanation, so that you knew it before you were through editing the piece.
It means that when the suicide of a BMX rider is splashed all over the news, that it’s come out that he had CTE (chronic traumatic encephalopathy) as a result of concussions related to his work over the years, you hold back from publication a piece on a close friend of his, an article in which the now deceased rider is also mentioned. And later, much later, when the piece does publish, you review it again, and when you see that it ends this way about the surviving friend —
In terms of his career, what’s he most proud of? That’s easy. “My longevity,” he smiles.
you rewrite that ending.
And it means if you’ve got a decidedly sensual portrait accompanying a profile, you don’t let it go to press with this quote associated with it: “I’ve always focused on kids because I had so much support . . . when you see kids that don’t have that, you want to do something to help them.” You make sure that something more appropriate to the image is swapped in. No kids involved.
And, yes, it also means knowing grammar and usage and punctuation. It means knowing the nuts and bolts of how sentences are constructed and how they cohere in longer passages. It means being able not only to feel that a sentence or a passage is off in some way, but to diagnose the problem as well, and in that diagnosis to have an available set of remedies.