MESI stands for Modified, Exclusive, Shared, and Invalid, which are states used in cache coherence protocols. These states help manage how multiple processors access and share data in a computer's memory, ensuring that all processors have a consistent view of the data. In a system using MESI, each cache line can be in one of the four states. The Modified state indicates that the data is updated in that cache but not in the main memory. The Exclusive state means the cache has the only copy of the data, while the Shared state allows multiple caches to read the same data. The Invalid state signifies that the data is not valid or up-to-date.