Emacs saves all the visited files from time to time (based on counting your keystrokes) without being asked. This is called auto-saving. It prevents you from losing more than a limited amount of work if the system crashes.
When Emacs determines it is time for auto-saving, each buffer is considered and is auto-saved if auto-saving is turned on for it and it has changed since the last time it was auto-saved. If any auto-saving is done, the message ‘Auto-saving...’ is displayed in the echo area until auto-saving is finished. Errors occurring during auto-saving are caught so that they do not interfere with the execution of commands you have been typing.
|• Recover:||Recovering text from auto-save files.|