Clarity is the quality of being easily understood, and in writing it means that each sentence says exactly what the writer intends, leaving nothing for the reader to guess.
In William Zinsser's 📝On Writing Well, clarity is the first of four principles of good nonfiction, alongside 📝Simplicity, 📝Brevity, and 📝Humanity. Zinsser ties it to thinking: a writer who has not decided what he or she means cannot write it clearly, so unclear prose usually signals an unclear idea rather than a lack of style.
The word comes through Middle English from the Latin claritas, from clarus, "clear, bright." It overlaps with simplicity but is not the same thing: simple prose can still be ambiguous, and a precise technical sentence can be clear without being simple. Ambiguity often hides in grammar rather than vocabulary, as in a notice that "we will upgrade our phones so we will be difficult to reach," or a request for "a list of all employees broken down by sex," where word order produces a meaning the writer never intended.
My note from the book: if it isn't clear, you might as well not write it. Clarity is the main prize, and almost like a law of physics, every sentence comes out just a little bit wrong.