This means you must know the best words, how to write good sentences, and how to organize your thoughts into effective paragraphs. This includes a good knowledge of English grammar, punctuation, spelling, capitalization, etc. Unfortunately, you cannot rely on friends to review your writing, unless they themselves are experts. Journal editors and copy editors also cannot be relied on to help you with your writing because they are too busy. You must give the time required for good writing.
Papers require editing, polishing and improvement before submission to a scientific journal.
Good writing requires that you be sincere in your writing. If you try to hide something from your readers or pretend to know something that you do not, this will show. Of course, good writing is a skill and it will improve with practice. The more writing that you do the better writer you will be. There are other ways to improve your writing as well.