Subtitles allow you to add a bit more detail and preface to your doc or page. To add a subtitle to your doc or page:
Hover over the first line of text (the title of your page) in your page's canvas.
Select "Page options." That panel should pop up on the right side of your screen.
Turn on the "Subtitle" switch.
Click into the prompt text under the title of your page to compose your custom subtitle.
You can add subtitles to as many pages and subpages of your doc as you like!