Rena is a term used in the context of Japanese adult content, particularly in hentai or anime porn. It refers to a specific genre of such content which features young, petite and usually schoolgirl-like characters. The tag "Rena" itself can mean that the video involves scenes with these types of characters, often dressed in school uniforms or other revealing outfits. This is a popular subgenre in Japanese adult entertainment due to its appeal among fans of anime and manga. In summary, "Rena" as a porn video tag denotes a specific type of content featuring small, young female characters typically in a school setting or similar themes.