BUILT-IN WOLFRAM LANGUAGE SYMBOL
is a notebook option that specifies whether the notebook should automatically be saved after each piece of output generated by evaluation in it.
Create a notebook with NotebookAutoSave->True and save it to a file:
Check the time of creation of the notebook file:
When you evaluate an input, the notebook is automatically resaved:
This shows the newer modification time of the file:
Close the notebook and delete the temporary file: