Which software engineering concept took you the longest to truly understand?

[Kirchhoff-]

Some concepts seem straightforward at first, but only truly "click" after years of experience.

For example:
Dependency Injection
SOLID principles
Immutability
Concurrency
Event-driven architecture
Composition over inheritance
Generics

Which software engineering concept took you the longest to truly understand?

What helped it finally "click" for you?