What is guanciale?
Guanciale is Italian cured pork cheek or jowl. It's traditionally used in classic pastas, like spaghetti all carbonara and bucatini all'amatriciana. Because it's largely fat, guanciale has a more seductive pork flavor and delicate texture than cured meat that comes from the belly (like pancetta, which is a common substitute, though the flavor isn't the same).
To make guanciale, you rub pork cheeks with some combination of salt, sugar, pepper, herbs and spices and then air dry them for several weeks. If you're feeling ambitious, try making your own. Otherwise, La Quercia, Buon Italia and Salumi Artisan Cured Meats are all good sources.
Reference: Guanciale, the magic Roman bacon (FX Cuisine)
Reference: How to prep pancetta (Cookthink)
Recipe: Fettuccine With Guanciale, Egg And Parmesan (Cookthink)