The MESI Protocol is a cache coherence protocol used in multiprocessor systems to maintain consistency among caches. It ensures that multiple processors can access shared data without conflicts by defining four states for cache lines: Modified, Exclusive, Shared, and Invalid. Each state indicates the status of the data in the cache and helps manage how data is read or written. When a processor wants to read or write data, the MESI Protocol checks the state of the cache line. If the data is in the Modified state, it means the cache has the most recent copy, and the data can be used directly. If it's in the Shared state, other caches may also have the data, and the protocol ensures that all caches remain synchronized.