For example if you have a data refresh file with Columns that looks like this:
                       Gender                        Prefix
Jane Smith      F                                 Ms.

John Smith 

Then any user that has nothing in the Columns, such as John Smith's gender and prefix would be overwritten with a blank.

If you have a  Data Refresh File with Columns that looks like this:
 

                       Gender                        Prefix                              Birth Date
Jane Smith      F                             Ms.

John Smith      M                            Mr.

Then those whole blank Columns will just be omitted in the Data Refresh and that Data would remain unchanged. Such as Jane and John's Birth Dates would not be changed.

Note: Usernames do not get overwritten by blank fields. Will data refresh change or delete an existing user's login credentials?