Collapsible/expandable headings like Google Docs has: by clicking on a small "arrow down" icon next to a heading, all content under that heading collapses (disappears) and reappears by clicking again on the arrow (which now has become an "arrow right"). Of course, when the content disappears, the remaining (still visible) doc content below that heading section moves up (to occupy the space of the disappeared content).
This allows to hide sections (e.g. on top) that might not be relevant (to the user) but can still occupy a lot of space and force the user to scroll down (where he/she might want to continue writing at the doc).