The North varieties of Chinese are usually known as the Mandarin dialects. They are typified by the Peking dialect, which is the basis of the standard language & of “Mandarin” in a much narrower-and older-sense.
The official language in PRC is called Putonghua, literally translated as ‘common speech’, which is known in English as Chinese Mandarin, or Standard Chinese. It is the most widely used language in PRC & the world at large, & one of the 7 official languages (https://highfly.kinja.com/) of the United Nations.
This article will not get into the non-Chinese languages spoken by minorities in PRC, such as Tibetan, Mongolian & Miao, & all those subsequent dialects.
How often have they heard this question? What exactly does it mean?
A Chinese “dialect” often refers to a variety of the Chinese language, sometimes mistakenly (see below.) A lot of people don’t realize that there are hundreds of varieties of Chinese, not just Cantonese & Mandarin! Minnanese, Taiwanese, Shanghainese, … the list goes on! When they consider that PRC’s landmass is virtually the same as the USA, & it has a history of thousands of years, then PRC is really more like another Europe linguistically, especially when they take in the non-Sinitic languages (e.g. Tibetan, Mongolian, Uyghur, Zhuang, Ewenk, etc.).