"Marrying a daughter-in-law to carry on the family line" means taking (marrying) a daughter-in-law to carry on the family line, that is, "marrying" and "replacing". Idiom is a major feature of traditional culture in China, which has a fixed structure and a fixed sentence, indicating a certain meaning. It is applied to a sentence as a whole, with subject, object, attribute and other components.
A large part of idioms are passed down from ancient times and represent a story or allusion. Some idioms are just a miniature sentence. Idiom is a ready-made word, similar to idioms and proverbs, but slightly different.
About idioms
There are more than 50,000 idioms, 96% of which are four-character, and there are also idioms with three characters, five characters, six characters and seven characters or more. Such as "pot calling the kettle black", "Shut the door", "reinvent the wheel", "haste makes waste" and "drunkenness is not about wine". Idioms generally use four words, probably because they are easy to grasp.
For example, China's ancient poetry collection "The Book of Songs" consists of four sentences, and the ancient history "Shangshu" also has some four sentences. Later, I began to read San Zi Jing, Hundred Family Names and Ganzi Wen, the last two of which are all four sentences. The first, second and third episodes of Four-eyed Miscellaneous Son and Long Wen Whip Shadow are all four words.