What are C++20 modules and how do they improve on headers?

Modules (C++20) are a new way to organize code that replaces textual #include with a compiled, importable unit (import mymod;). They avoid repeatedly re-parsing headers (faster builds), don't leak macros or private details, remove include-order fragility, and give better encapsulation. Adoption is gradual as tooling matures, but modules address longstanding header/preprocessor problems.

export module math;
export int add(int a, int b);
// user: import math;