These four characters combine to create a dynamic metaphor. The carts and cars are as numerous and ceaseless as a flowing river, and the long line of horses and vehicles winds down the street like a powerful dragon. The imagery is not of being stuck, but of a grand, impressive, and continuous flow.
`车水马龙` originates from a historical text, the History of the Later Han (后汉书), describing the constant stream of visitors arriving in fancy carriages at the home of a powerful general. From its origin, the idiom has been associated with prosperity, popularity, and importance. A place described as `车水马龙` is a place where things are happening—a hub of commerce, culture, or power. A useful comparison for Western learners is the phrase “bumper-to-bumper traffic.” However, their connotations are very different. “Bumper-to-bumper” is almost exclusively negative, implying frustration, delay, and the stress of a traffic jam. In contrast, `车水马龙` is often neutral or positive. It can be a simple observation of a busy street, but it frequently implies a sense of wonder at a city's vibrancy or a business's success. It celebrates the Chinese cultural value of `热闹 (rènao)`—a lively, bustling atmosphere that signifies good fortune and prosperity. A quiet, empty street might be peaceful, but a `车水马龙` street is alive and thriving.
As a `chengyu` (four-character idiom), `车水马龙` is considered more literary and is often found in written Chinese (articles, books, descriptions) or in more formal or descriptive speech. You wouldn't typically use it in a casual complaint to a friend about your commute.