The MOESI cache coherence protocol is a system used in multiprocessor computer architectures to maintain consistency among caches. It stands for Modified, Owned, Exclusive, Shared, and Invalid, which are the five states a cache line can be in. This protocol ensures that when one processor modifies a data item, other processors are aware of this change, preventing stale data from being used. In the MOESI protocol, each cache line can transition between these states based on operations performed by the processors. For example, if a processor modifies a cache line, it changes to the Modified state, while other caches holding that line must update their status to Invalid. This mechanism helps improve performance by reducing the need to access slower main memory.