From this menu, you can select one of three modes.
Editing mode: Lets you edit the document directly. This is the mode you're in by default.
Suggesting mode: Lets you suggest edits to a document that other people can review. Suggestions that are accepted will be added to the document, while ones that are rejected are deleted.
Viewing mode: Lets you view the document read-only, which will prevent you from changing anything until you return to another mode.