The cache hit rate is the percentage of requests that result in cache hits. In Varnish it is evaluated as cache_hit / (cache_hit + cache_miss). The higher the hit rate the more effective your cache is. For example if the hit rate is above 80%, it means the majority of requests are fulfilled by an object already present in cache and no backend request is made. This, of course, makes the response time shorter and reduces or completely avoids the chance that your backend will be overwhelmed.