The porn video tag "konosuba-aqua-hentai" refers to a hentai (Japanese adult animation) video that involves characters from the popular anime series "Konosuba." Specifically, it features Aqua, one of the main characters in the show.
Konosuba is a light novel series and an anime that tells the story of Kazuma, a high school student who dies and is given the chance to go to another world. He assembles a party consisting of a goddess, a wizard, and Aqua, a maid and deity of water, to fight evil forces.
In this hentai video tag, "aqua" refers to Aqua from Konosuba. The term "hentai" signifies that the content is adult in nature and consists of explicit and often fantastical sexual situations involving the character Aqua. This tag would be used by adult fans of the series who are specifically interested in explicit content featuring Aqua.