One of tags used for Hetalia-related fanarts.
Works with this tag always show Ludwig and Berwald together in them.
"旦那(Danna)" in this case means "husband" in Japanese, so this tag might can be translated like this; "The Husbands".
Why the two characters are called that way is because of the groupings shown below;
お花夫婦("The Flower Couple")
and 花たま夫婦("The Hanatama Couple")
Those two pairs tend to be considered like somewhat sort of man-and-wife by many of Hetalia fans, and Ludwig and Berwald are more likely to be considered as "husbands".
Here is the image which both "couples" shown together.