I wonder what is .api file for? For example: https://github.com/sebaslogen/resaca/blob/main/resaca/api/resaca.api

Preserving binary compatibility. Basically, API files usually have the JVM signatures of the declarations you have, and so it helps to ensure that your library didn't change those signatures. Preserving those signatures allows users of the library to use a new version of it without recompiling their code, thus gaining improved performance, for instance.

Thanks for the explanation! Is the "preserve binary compatibility" correct keyword for me to google more details?

Yes that should give more info, also ABI (application binary interface)

https://github.com/Kotlin/binary-compatibility-validator generates the .api files and checks the current API against the existing .api dumps.

There's a section in the Kotlin docs about binary compatibility https://kotlinlang.org/docs/jvm-api-guidelines-backward-compatibility.html#tools-designed-to-enforce-backward-compatibility