You have not applied pending changes to the currently selected configurations.

The Microsoft development environment cannot change the configuration (either debug or release) while there are changes that are unapplied from your current dialog box. If you choose Yes, the changes are applied to the new configuration you selected. If you choose No, the configuration is changed to the one you selected, but all the changes in the dialog box are dropped. If you choose Cancel, you are returned to your original configuration with all the changes still unapplied in the dialog box.