Disclaimer: this is based on my own experiences as an editor for an International website that accepts stories from their membership, and may not mirror that of the editors of House of Honor.
You may have spent weeks or even years hammering out a world of your own making, and you have an interested publisher! You just need to send them your manuscript, right? Wrong.
There seems to be the erroneous idea that editing and proofreading start with the publisher, in truth, it should begin with you, the writer. The manuscript should be sent off (or even offered) when you have done everything you can to ensure it is perfect. Don't worry, it won't be. No one expects you to be an English major, or perfect - however neglecting to give it your best shot can result in rejection; or at the very least slow down the publication of your book.
As an editor, I can honestly say that I can usually tell within the first few paragraphs if the writer even bothered to objectively proofread their work before submitting. Sometimes it is bad to the point of not only giving me a migraine but making my eyes bleed. And I'm not talking about technical syntax, but basic structure and punctuation. Lately, there seems to be a love affair with the ellipsis (...) instead of any other punctuation. That, in my view, is as bad as no punctuation (never mind the fact it's being used improperly).
Another very common error is the run-on sentence. Run-on sentences contain too many ideas without proper punctuation. This is evil because they can make ideas less clear and confusing to the reader. It's a lot like listening to someone talk really fast without pausing for breath. There are 14 punctuation marks commonly used in writing, how about using a few of them?
People generally assume that an editor is a spell checker, and a stickler for grammar, however, there's a lot more to it. An editor's job is to polish and refine a story, and to aid you in making changes that will keep the audience pulled in and interested; and, hopefully, help you grow as a writer.
This is where revisions come in, and that can be a rather emotional thing to go through. Writers have a strong personal relationship with their manuscript, whereas the audience doesn't and are very quick to judge. A good editor will suggest changes that will improve your story, without changing your voice. Logically, if I'm not stopping to correct simple errors in structure and punctuation, I'm able to get to the meat of your story quicker. This is the longest part of the publication process and can be expedited by simply self-editing prior to submission; which translates into seeing the finished product sooner.