A mouthful is a large amount of food that you swallow all at once.
When we talk about something being a mouthful, we usually mean it's quite big or difficult to eat. You might say that a huge burger was a mouthful because it took several bites to finish. Or maybe you're trying to describe how hard it is to drink from a giant cup, and you'd say the liquid is a real mouthful - it's too much for one swallow. Sometimes we use this phrase in a more playful way, like if someone says something really complicated or confusing, we might call it a mouthful because it's hard to understand all at once.
The amount that will fit in a mouth.
Example: He swallowed a mouthful of sea water when he fell in.
