From time to time, Emacs automatically saves each visited file in a separate file, without altering the file you actually use. 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, it considers
each buffer, and each is auto-saved if auto-saving is enabled for it
and it has been changed since the last time it was auto-saved. When
auto-save-no-message variable is set to
default), the message ‘Auto-saving...’ is displayed in the echo
area during auto-saving, if any files are actually auto-saved; to
disable these messages, customize the variable to a non-
value. Errors occurring during auto-saving are caught so that they do
not interfere with the execution of commands you have been typing.