Last modified allows you to display the date and time of the last modified made to the record in the format YYYY-MM-DD HH:MM:SS.
This field cannot be modified, which allows, among other things, to keep a record.
Last modified are displayed only from the creation of the column. The modifications made before the creation of the column will be empty.
To have more information about the changes made in a column, you have to use a column of History