Set this toggle to On and your progress will be saved every 15 seconds. That way, if you leave your message to work on something else, you can come back later and start off right where you left off.
If you are experimenting with the editor and don't want to keep track of the changes you've made, feel free to set your toggle to Off.
Click this button to manally save your progress. Make sure to do this if you have your Auto-Save set to Off.