Memory consistency refers to the rules that determine how operations on shared memory are perceived by different threads in a computer system. It ensures that all threads see a consistent view of memory, meaning that once a write operation is completed, all subsequent read operations will reflect that change. This is crucial in multi-threaded environments to avoid confusion and errors. Different memory consistency models, such as Sequential Consistency and Release Consistency, define how and when updates to memory become visible to other threads. These models help developers understand the behavior of their programs and ensure that they can predict the outcomes of concurrent operations reliably.