MOESI is a cache coherence protocol used in multiprocessor systems to maintain consistency among caches. The acronym stands for Modified, Owned, Exclusive, Shared, and Invalid, which represent the different states a cache line can be in. This protocol ensures that when one processor modifies a data item, other processors are aware of the change, preventing stale data from being used. In a MOESI system, each cache line can transition between these states based on read and write operations. This helps optimize performance by reducing the need for frequent memory accesses, allowing processors to work more efficiently with their local caches while ensuring data integrity across the system.