"Hentai" is a term used to describe Japanese adult animation or manga, often featuring fantastical themes and characters with exaggerated features. The term hentai itself translates to "pervert" or "perverted", but in the context of pornography, it specifically refers to this style of animated content.
As for "HD," this stands for high definition, meaning that the video quality is higher than standard definition, providing more detail and clarity for viewers. In this case, "hentai-hd" would refer to high-quality, high-definition Japanese adult animation or manga.