Moho's Note layers work kind of like sticky notes. You can add a note layer to remind yourself of something you need to work on, or to explain to other animators some important aspect of your project file. If you add a note layer at a frame other than zero, the note will only appear starting at that frame. That way, you can add notes that refer to specific parts of your animation (ex. "Use more exaggeration here!").
By default, Note layers do not appear in final rendered output, so your audience doesn't need to see your comments to yourself. However, if for some reason you do want a note to appear in the final product, you can do that by un-checking "Don't render this layer" in the Layer Settings dialog.