What is RAII in C++ and why is it important?

RAII (Resource Acquisition Is Initialization) ties resource lifecycle to object lifetime. Acquire resources in constructor, release in destructor. When object goes out of scope, destructor automatically releases resources. Examples: smart pointers for memory, file handles, mutex locks (std::lock_guard), database connections. Benefits: exception safety (destructor called during stack unwinding), no resource leaks, deterministic cleanup, cleaner code (no explicit cleanup). RAII is fundamental C++ idiom. Standard library extensively uses RAII: fstream, unique_lock, shared_ptr.