In a CDN, a cache miss occurs when the requested content is not present in the cache memory of the CDN edge server. In this situation, the CDN edge server must retrieve the content from the origin server, which can result in increased latency and slower load times for the user. Cache misses can occur when a requested content is not frequently accessed, or when the content has expired in the cache memory. CDNs are designed to minimize cache misses and improve content delivery by caching frequently accessed content at edge locations.