A memory management unit (MMU) is a hardware component in a computer that handles the mapping between virtual memory addresses and physical memory addresses. It allows the operating system to use memory more efficiently by enabling features like virtual memory, which lets programs use more memory than is physically available. The MMU also provides memory protection, ensuring that one process cannot access the memory space of another process. This is crucial for system stability and security, as it prevents unauthorized access and potential data corruption among different running applications.