The Compatibility Checker found one or more customization-related compatibility issues.
Important: Issues that cause a minor loss of fidelity might or might not have to be resolved before you continue saving the workbook—data or functionality is not lost, but the workbook might not look or work exactly the same way when you open it in an earlier version of Microsoft Excel.
Issues that cause a minor loss of fidelity
This workbook contains a customized Quick Access Toolbar and/or custom user interface parts that are not supported in earlier versions of Excel. These custom features will not be available in earlier versions of Excel.
What it means Because the ribbon user interface in Excel 2007 and later is very different from the menus and toolbars in Excel 97-2003, any Quick Access Toolbar customizations that were made in Excel 2010 are not available in Excel 97-2003.
What to do In Excel 97-2003, you can add similar custom commands to toolbars and menus.