Both 元 (yuán) and 块 (kuài) are used to refer to money in Chinese.
元 (yuán)
元 is the formal term for the unit of currency in China, officially known as the Chinese Yuan.
It is used in formal contexts, such as in financial documents, official communications, and when writing amounts of money in formal settings.
e.g.
一百元 (yì bǎi yuán) 100 yuan
我们的这份合同价值300万元。(Wǒmen de zhè fèn hé tóng jiàzhí 300 wàn yuán.) Our contract is worth 3 million yuan.
这个国家的人均GDP很低，才三千元。(Zhè ge guójiā de rénjūn GDP hěn dī, cái sān qiān yuán.) The per capita GDP of this country is very low, only 3,000 yuan.
块 (kuài)
块 is a more informal, colloquial term for the same unit of currency. It’s similar to saying “bucks” or “dollars” in English.
It’s often used in everyday conversation or when speaking casually about money.
e.g.
一百块 (yì bǎi kuài) 100 bucks
我想借你500块钱，可以吗？(Wǒ xiǎng jiè nǐ 500 kuài qián, kěyǐ ma?) I want to borrow 500 bucks from you. Is that okay?
这里的公交车很便宜，只要一块钱。(Zhè lǐ de gōngjiāo chē hěn piányí, zhǐ yào yī kuài qián.) The bus fare here is very cheap, only 1 yuan.
Summary
元 (yuán) is the formal, standard term, while 块 (kuài) is the informal, conversational term.
