Rule 34 is a popular meme that essentially states, ‘if it can be thought up, there is porn of it.’ In other words, it’s the idea that, since the world is filled with weird and perverse people such as yourself, there is porn of every single thing imaginable. Or, as rule34.paheal.net puts it in their About section, “There IS porn of it, no exceptions.”
Intricate drawings of Pokemon fucking their masters? There’s porn of it. Weird manga of fairies having every orifice filled with monstrous tentacles? There’s porn of it. Videos of grannies drinking shit and cum cocktails? Well, you get the picture (rather unfortunately in that case).