成語 [成语] (chéngyǔ) are Chinese idioms usually consisting of four characters. They tend to pack a lot of meaning into those four characters and many have a story, myth or moral behind them from Classical Chinese literature, in which they were used extensively. If you’re not familiar with the stories, it will be very difficult or impossible to work out what the idioms mean. They’re still quite commonly used in modern written and spoken Chinese, and there are between 5,000 and 20,000 of them.

Here are a few examples:

一日千秋 (yírìqiānqiū) = “one day, a thousand autumns” – implies rapid changes; one day equals a thousand years

一日千里 (yírìqiānlǐ) = “one day, a thousand miles.”- implies rapid progress; travelling a thousand miles in a day

一日三秋 (yírìsānqiū) = “one day, three autumns.” – when you’re missing someone very much, one day can feel as long as three years.

From Wikipedia

A good place to find out more about chengyu is this site, which explains a number of them in Chinese and English. Another useful chengyu site is this one, which contains a dictionary of 13,000 of them with explanations in Chinese.

These idioms are also used in Japanese and are called 四字熟語 (yojijukugo) – four character idioms. They come mainly from Classical Chinese and have the same or similar meanings to the Chinese ones. A dictionary of Japanese four characters idioms, with explanations in Japanese, can be found here, while this site explains some of them in English.