在于 zàiyú to rest with

to rest with; to lie in; to be due to (a certain attribute); (of a matter) to be determined by; to be up to (somebody)

怎么记住“在于”: 在 at + 于 at = to rest with.

汉字

在于 (zàiyú) is a formal linking verb meaning "to lie in / to consist in," used to state where the key point of something rests.
Classic pattern: A在于B - 成功在于坚持 (chénggōng zàiyú jiānchí) - success lies in persistence.
It's written/argumentative register - essays, speeches, explanations of cause - not something you'd casually say about small talk.
Similar to 取决于 (qǔjuéyú, to depend on), but 在于 pinpoints "the essential factor is," while 取决于 frames an outcome as contingent on a variable.