Pizza is a flat, bread-based type of food found on Earth.
The base of the pizza is a round disk of flattened, cooked bread dough; numerous things are piled atop it; tomato sauce and cheese are almost universally common, but several other types of toppings can be added. Pizza was invented in Italy many centuries ago.
Role in Invader Zim
Pizza is Gaz's favorite food, and is the main food product sold at Bloaty's Pizza Hog. In fact, pizza was the main focus of the episode of the same name. GIR likes pizza too, while Zim seems to detest it, or at least the smell of some of the cheeses used to make it.
Several Irkens are seen eating what appear to be Irken pizzas in "Hobo 13". It's unknown how they compare to the Earth-made variety.
In Invader Zim: Enter the Florpus, GIR tries to convince that Phase 2 of his new plan (which Zim has forgotten) involves making a million pizzas and letting GIR roll around in them to become a giant pizza. Zim refuses this, his response suggesting that GIR has brought it up before. Later, when going through all experiments in the base to try and figure out Phase 2, GIR presents Zim with several pizzas, possibly in reference to this idea.