Reverting Files File Aliases
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 that 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 been changed since the last time it was auto-saved. The message
Auto-saving...' is displayed in the echo area during auto-saving,
if any files are actually auto-saved. Errors occurring during
auto-saving are caught so that they do not interfere with the execution
of commands you have been typing.