A mouthful is an amount of some substance that can fit in a person's mouth. If you eat a mouthful of chocolate cream pie, you consume a bite of it.
A mischievous kid might spit a mouthful of water at her sister, while a thirsty athlete might gulp a mouthful of water after her workout. In both cases, the mouthful is as much water as can fit in their mouths. If you're not very hungry for dinner, you might just have a few mouthfuls of mashed potatoes — just a few bites, in other words. Another kind of mouthful is a long, complicated word, phrase, or sentence.