There are between six and twelve main regional groups of Chinese (depending on classification scheme), of which the most populous by far is Mandarin (c. 800 million) which is required learning in schools, followed by Wu (c. 90 million), and Cantonese (c. 80 million). These are different enough to be mutually incomprehensible. This means that a person speaking Cantonese will probably not be able to understand one speaking Wu.

The most interesting thing, though, is that the written language is the same, regardless of the dialect. So someone going from one side of China to the other, and into a region where they can neither understand or be understood verbally, will often simply write down the idea they want to get across. It can then be understood.