It is slang, used to express something that has great appeal to you. Porn is short for pornography. So, food porn is food that looks delicious to you

I thought of this earlier, and was puzzled. Then, just now, I remembered. These phrases refer to magazines with color pictures. Thus, Car Porn refers to Car Magazines. Food Porn refers to cooking and recipe magazines, with lots of pictures of food.

It's a joke. "Porn" is short for "pornography". When people say "food porn", they're joking that the food looks so delicious it's like looking at porn. A person who really likes cars might joke that a photo of a really good car is "car porn".