A frog's gullet is its throat or esophagus; this is the passage from the mouth to the digestive system. If a frog's digestive process is seen as a series of steps, passage of food from the mouth to the gullet can be seen as the second step in digestion.
Frogs aren't the only animals with a gullet; this is a general term to refer to an animal's esophagus. In frogs, the opening to the gullet is visible in the animal's wide mouth. The gullet is a part of the frog's digestive tract and a part of its vocal system. The "ribbit" croaking sound frogs makes originates in this part of the frog's body.