CSV is comma separated values format. A lot of data comes in that format. When you made changes in excel, it cannot save things that aren't comparable with the CSV format (functions, formatting, etc.). If you choose save anyway on that popup, it will save it as a CSV, which will keep the data, but lose all of the other items. If you want to keep it as an excel file, choose File> save as... and underneath the file name, choose excel workbook (*.xlsx)
