Generally a collection of food images, appealing enough to post on social media. It can be a picture of your egg-sausage-hashbrown breakfast, or even some leftover spaghetti-and-meatballs. Not actual porn, sorry.
Person 1: Why the hell is "Food Porn" in your search history?
Person 2: Oh, I was looking at some pepperoni pizza the other day.
Person 1: Wh-what?
Person 2 shows a picture of an actual slice of pizza.
Person 2: Yeah, you thought! Do you have some dirty mind or something?
Person 1: Don't we all?