"Gay kissing" is a term used in the context of adult content to describe two men engaging in intimate, passionate kissing. In a porn video tag, it signifies that this scene or video will feature homosexual actions between male actors. It's an essential aspect of gay adult videos and helps viewers identify if they are looking for such content. The term "gay" is used here as an adjective to describe the nature of the act being performed, not the genre of the video itself.