Easy reading is damned hard writing.
(Please excuse the curse word. It may indicate Hawthorne’s frustration with the work necessary to produce good writing, or it may indicate the strength with which Hawthorne believes this.)
The point of this quote is that clear, easy-to-read writing is not easy to produce. Instead, it is the result of writing, analyzing what you write, and re-writing—perhaps many times.
When you write, you are attempting to communicate. The more work you put into writing, the better you will be able to communicate. Hard work by you leads to easy understanding by your reader.
People have told me, “Writing is so easy for you.” This isn’t true. I have practiced writing, studied writing, and analyzed what makes writing clear. The documents they read are the result of much work: writing, criticizing, and rewriting until they are “easy reading.” That’s what great writers aim for: not easy writing but easy reading.