Compulsory misses occur in computer systems when data is requested from memory but is not found in the cache. This situation arises because the data has never been loaded into the cache before, often due to it being accessed for the first time. As a result, the system must retrieve the data from the main memory, which is slower than accessing it from the cache. These misses are a normal part of cache operation and are expected when a program starts or when it accesses new data. Understanding compulsory misses helps in optimizing cache performance and improving overall system efficiency, particularly in relation to cache memory and computer architecture.