A basic tool of Mexican cuisine. It is a big, flat, handleless pan.
Originally made of clay, it is now commonly metaly (usually iron).
A comal is useful for heating up the tortillas, making quesadillas (and sopes and tlayudas ...) and roasting chiles. In a pinch, it can also be used to toast bread.

advice: in buying a comal, the single most discriminating factor is thickness. A thin comal, much like a cheap pan, will have hot and cold spots and will warp over time. Go at least for 1/8". The clay ones are more fragile and heavier, but they are the best.
Size also matters. A big comal will allow you to make four quesadillas at the same time, or heat up four tortillas. The latter is more important, since tortillas lose heat fast and cold tortillas are an abomination unto the eyes of Benito Juarez.