Have you ever wondered why we describe the sound a frog makes as "ribbit"? Chances are, you've never heard a frog actually say "ribbit." Why is that?
Well, there are hundreds of different kinds of frogs, and they each make their own unique sound. It's not so different from how humans make sound. Frogs make air rush over their vocal cords, and when the cords vibrate they produce sound.
It's loud because frogs have pouches of skin called vocal sacs under their chins. After air passes over the frog's vocal cords it goes into the vocal sac which helps project the sound. This is why you sometimes see frogs with these huge bubbles jutting out under their chins. Frogs have their own built-in amplifiers.
And they croak because they're in love. Frog croaks are mating calls. It's usually the male frog that sends out the call. Now you're probably wondering how the female frogs can tell which croak belongs to which frog. Well, the frog ear, or tympanum is specially shaped to amplify croaks from their own species and mute calls from other kinds of frogs. And if they like what they hear, they find the croaker and make some tadpoles.