If The Active Child Theme is already HTML5 enabled
By activating this HTML5 Markup option you will be enabling a Genesis framework feature that outputs HTML5 markup in place of the old XHTML tags.
Please Note: Since the enabling of this feature changes some of the HTML markup of Genesis you may find that some of your Custom CSS, if you have added any at this point, may need to be tweaked to maintain the design changes made by such CSS. In light of this you may find the following link useful: GENESIS XHTML TO HTML5 CSS CONVERTER
Also Note: The "Hook" setting for both Custom Widget Areas and Custom Hook Boxes (found in the "Genesis Extender Custom" admin section) have both "HTML5 Content Hooks" and "Pre-HTML5 Content Hooks". When this HTML5 Markup option is enabled use the "HTML5 Content Hooks". When it is disabled use the "Pre-HTML5 Content Hooks".
Finally: If you see the message " Note: The Active Child Theme is already HTML5 enabled." then this means that the currently activate Genesis Child Theme already has HTML5 enabled and therefore this option does not need to be enabled for the Genesis HTML5 markup and features to be active.