相xiāng辅fǔ相xiāng成chéng

Literal: mutually assisting, mutually completing
Figurative: To complement and reinforce each other, so that each part needs the other to work at its best; to go hand in hand.

Where does 相辅相成 come from?

It does not come from a specific anecdote; it is a set phrase of written Chinese with a parallel structure, like 相亲相爱. 相 marks the action as reciprocal, "each other"; 辅 means "to help, to support," as in 辅助, and 成 means "to complete, to bring to fruition." Together they say that two things do not merely coexist but each makes the other possible. It is usually applied to abstract pairs (theory and practice, work and rest, reading and writing), and it is very common in essays, speeches and opinion pieces.

How is 相辅相成 used? Two examples

工gōng作zuò和hé休xiū息xi是shì相xiāng辅fǔ相xiāng成chéng的de，累lèi垮kuǎ了le反fǎn而ér什shén么me都dōu做zuò不bù好hǎo。
Work and rest go hand in hand; if you wear yourself out, you end up doing nothing well.
学xué语yǔ言yán时shí，听tīng力lì和hé口kǒu语yǔ相xiāng辅fǔ相xiāng成chéng，最zuì好hǎo一yī起qǐ练liàn。
When you learn a language, listening and speaking reinforce each other, so it's best to practice them together.

In Wanyu, all 400 chengyu have audio on every sentence, and you can save them to your decks and review them with spaced repetition. Free.