Josh Marshall says

The essence of all good writing is clarity. Style seems like a separate attribute of good writing. But it’s not. Style is really just a byproduct of clarity and concision. It is the personality or other uniqueness of the writer coming through on the page because they write clearly.

So how does one write clearly? The writing is the easier part of it. Once you know precisely what you mean to say, writing it is usually straightforward if not always easy. At least 90% of poor writing stems from the writer not knowing exactly what it is they mean to say. We’re all lazy like this. Half-formed thoughts pop into our heads and we push them out as words that have some relation to the hazy ideas and feelings in our minds. This may do in talking to your coworker or spouse about simple topics over the course of the day. The points are simple. In speaking we have physical cues and intonation. If you’re not clear the first time you can try again.

Writing is different. If you are writing it down the ideas must be significant or else you wouldn’t be writing them down. You only have one shot to make your meaning clear. There is no follow-on interaction to fill in the gaps. Often what you mean to say is still more a feeling than a thought or a not fully worked through set of ideas and connections between them. Jargon and vaguenesses are added to the mix to cover spots in the writer’s thinking that aren’t clear in their own head. Or they paper over things the writer means but is not ready to say.

Take a wordy or clumsy sentence you may write. Examine it and you will almost always see that it is wordy or clumsy because the idea is unclear in your head. Fuzzy parts of your thinking, connections that don’t fully bear out or don’t connect in a clear way end up on the page in fuzzy or vague groupings of words. If you work at the idea in your head long enough that you know exactly what it is, precisely how one idea or action connects to the idea or actions that came before and after it, the language can be direct, brisk and clear. It all but writes itself … once you know precisely what you mean to say. Absent that clarity it never can because the language you use to express your ideas can never be clearer than the ideas or thoughts as they exist in your mind. Work over the ideas, how each connects to each other, the order and progression that connects them and the words will, largely, take care of themselves.

Clarity is simply taking the meaning in the writer’s head and conveying it as clearly as possible in words.