A mouthful of drink or food is the amount that you put or have in your mouth.
e.g. She gulped down a mouthful of coffee...
e.g. Chew each mouthful fully before the next bite.
If you describe a long word or phrase as a mouthful, you mean that it is difficult to say.
e.g. It's called the Pan-Carib-bean Disaster Preparedness and Prevention Project, which is quite a mouthful.
它被称为 the Pan-Caribbean Disaster Preparedness and Prevention Project（泛加勒比备灾防灾项目），名称相当冗长拗口。