Cache coherence refers to the consistency of data stored in multiple caches within a computer system. When multiple processors access shared data, they may store copies in their local caches. If one processor updates its copy, other processors must be informed to maintain consistency, preventing them from using outdated information. To achieve cache coherence, various protocols are employed, such as MESI (Modified, Exclusive, Shared, Invalid). These protocols help manage the states of cached data, ensuring that all processors see the same value for shared variables. This is crucial for the correct operation of multi-core processors and parallel computing systems.