Docs will be the place where you'll spend most of your time, doing the majority of your work.
The best way I could describe it: Docs is Google's Word equivalent. It can pretty much do most of the things Word can and in addition it is very handy with the Comment and Suggestion modes that will make your life a lot easier when you interact with someone to better your story.
You can add a comment by highlighting the area of the text you'd like to comment on which will make the + marker on the right side of the document. Click on it. Comment and It'll give you a neat little comment bubble with the option to Reply or Resolve. Here you can have small discussions about the segment in question.
In the upper right corner of Docs you'll see a drop-down menu.
In the default state if you are in your own document at the time, it'll show "Editing" mode and if you are invited to read or edit someone else's work you can check here if you are allowed to make suggestions.
Making suggestions can be a powerful tool because it allows you or your fellow author to directly offer replacement options for a segment, or add entirely new segments to your writing, making cooperative work much easier
When you added a suggestion, it'll be highlighted differently from comments, but still will give you the opportunity to have a bit of conversation on the suggested addition or change, which you can either accept or reject eventually.