The Story So Far...
So, we've covered research and plotting where we hammer out the very basics of what will happen in the book without actually writing the book. Then we talked about outlining, which is quite a bit like the first step but we go through each character and each scene until we have a basic storyboard.
And then the fun (and frenzy) of writing the first draft, tearing it apart, rewriting... and then, once every single plot hole is smoothed over and filled in, and the big issues are fixed we move onto the various stages of editing.
...On to the final stage of actual editing.
Close editing is probably the stage of editing everyone thinks of when someone says they have an editor because this is that step you're expecting an editor to be fully involved in.
You're not wrong on that latter bit, but as you can tell there is more than one kind of editor.
A Close Editor is an editor that, after the development and plot edits have been done... and after all the stylistic edits have been done with words and sentences being tugged back and forth until each word has been picked for maximum impact... fixes the grammatical and spelling errors, to a set style guide laid out by the publisher (or in use by the target market) so that readers aren't picking out misspelled words or other issues.
While some could argue that much of the Close Editor's job can be replaced by Grammarly or other editing software, the reality is that nothing can replace a professional human being as that second set of eyes.
However, that doesn't mean you shouldn't use any tool you can put into your writing toolbox to make that human being's job easier. The first benefit is that you will write better in the end, which means anything done at this point can only be that much better.
A close editor also can look a bit at style as well. If they're picking up on word use and grammar that means they're also having to tweak the style a bit too (and vice versa).
I won't lie.
This stage, while mostly hands-off by the write unless the editor needs to ask something, takes the longest of all of them. That one set of eyes has a monumental task and they can only do so much of it before they run into 'eye fatigue' and have to set your manuscript down.
Think of it like being asked to put a million grains of sand into the pattern of a snowflake with only a set of micro-tweezers while looking through a microscope, or anything else requiring that level of focus on something tiny in the middle of something huge... and then numerous tiny things until the whole thing is done.
That's close editing.
I tend to hire two close editors, and before I send anything to them I'm also using any electronic or another tool to double-check my own work. It's not that I don't have faith in any of them (I do), but when you're looking at a manuscript sitting at 100k words, everyone encounters fatigue. Between the three of us, it ensures the greatest chance of that manuscript being as error-free as humanly possible.
Tools I suggest:
Microsoft's built-in grammar/spell-check set to the localization of your target market
Yes, this post was short. There's not a whole lot to be said here.
After this third draft has passed through editing, it's on to the last step before publishing.