A DD is short for "Daily Deviation" ... Every day the CVs (Community Volunteers of deviantART) select a group of works deserving of being shown in a feature that is seen by the WHOLE website!!! ... So when your work is chosen, they put the banner you have there (the one that says "Daily Deviation") ... You can read a little more about DDs here ---> DD-Suggesters, a group I'm currently working on ... it's still under construction, but it already contains a lot of information about this topic!
So again my dear, congratulations!!! Your work is wonderful!!!