Yegor applies a kitchen metaphor to code:
The kitchen is clean when there is no dirt in the oven. But if its electric panel speaks French, I can’t use the kitchen. Even if it’s perfectly clean. It’s not clear how to use it—that’s why it’s useless.
Sounds good to me, but how do you know if your code is actually clean and clear? He provides a heuristic:
If a stranger can modify your code and fix a bug in less than an hour, it is maintainable.
The entire post is well worth a read.