This question appears when you change the type of color series being use to Default color sequence for all graphs.
Before you select this option, some color series is being used for you graph. You may have made customized it in various ways. Now that you want to go back to using the global default, is your intention that you want to dump your current customizations and go back to using the global default? Or in your intention that your customizations should become the new global default?
If you select Yes, the customizations you've made to the color sequence for this graph (or template or index) will be dumped. The colors you see in the dialog will change to display the colors in the global default series.
If you select No, the custom colors currently showing in your dialog will become the global default colors (when you press OK). All graphs that don't have custom colors defined will start using these colors. If you have screwed up the color scheme, you don't want to select this.