I did a class awhile ago on clarity. We covered using the right word, making sure your sentences have the correct subject, your commas are in the right place, and that your characters are coming off the way you intended. Basically, instead of looking at things from a right or wrong standpoint, we covered how to use your tools to get the reader to have the reaction or understanding that you intended them to.
An author spends a lot of time in their story. They write, and re-write, edit, revise and tear down and re-assemble it. Often they have most of it memorized by the time they’re through. Also often, they can get a little fuzzy about what is and isn’t actually down on the page.
Because we know so much about our story, about our characters and our world, it is easy to think that something is obvious when, in fact, it isn’t. It’s very easy to think that you have made something clear when the reader is not privy to a significant bit of information that would allow them to “get it.” Keeping in mind that the reader will not have any knowledge that we do not lay out on paper for them is harder than you might think.
We never want to over explain, but under explaining is an equally obnoxious mistake. (and one I am often guilty of!)
If something is explained in passing on page four, will the reader to remember it exactly on page 250? Some readers will, certainly, (super readers!) but a dash of reminder or subtle hint is often needed to bring your carefully lain fact back into the foreground for application.
As the author, you have insider information. It can be hard to know for certain whether your intent is working and your meaning is clear. But it is worth devoting some time to, in particular during revisions. "Am I saying what I mean to be saying," deserves at least one good pass through on its own.
And of course, viva la beta reader who can stop us short and say, HUH?
or "you keep using that word...I do not think it means what you think it means."