Did you know there are 3 modes with which to view your Google Documents? As long as you have editing rights to the document, you can move freely between the three modes. They each have a specific purpose.
If you look at the top right corner of your Google Doc, you will see the word "Editing" with a little pencil icon next to it. By default, you are put into editing mode when you create a new Google Doc. Editing mode is exactly what it sounds like, it allows you to edit your document.
Suggesting mode is when you are suggesting changes to someone else's document you are looking over or working on collaboratively. Once you are in this mode, you can type directly into the doc and it will track your changes in green. It then puts in a comment letting others know what you are suggesting. They can click to accept your changes or comment back to you.
Viewing mode is just how it sounds - it gives you view only access to the document. This is great for testing links, table of contents, bookmarks, etc.
There you go! All three modes explained.