What is response caching in Apigee and how do you configure the ResponseCache policy?

<!-- ResponseCache policy -->
<ResponseCache name="ResponseCache">
  <!-- How to build the cache key -->
  <CacheKey>
    <Prefix>weather</Prefix>
    <!-- Cache key includes the city query parameter -->
    <KeyFragment ref="request.queryparam.city" />
  </CacheKey>
  <!-- How long to cache the response -->
  <ExpirySettings>
    <TimeoutInSec>300</TimeoutInSec>  <!-- 5 minutes -->
  </ExpirySettings>
  <!-- Skip caching if the client sends Cache-Control: no-cache -->
  <SkipCacheLookup>request.header.cache-control = "no-cache"</SkipCacheLookup>
</ResponseCache>

<!-- Placement: ResponseCache appears in BOTH:
  1. ProxyEndpoint PreFlow Request   (to check for cache hit)
  2. ProxyEndpoint PostFlow Response (to populate the cache on cache miss)
  Apigee handles the dual role automatically with this single policy -->

<!-- Flow variables after policy executes:
  responsecache.ResponseCache.cachehit    -- true/false
  responsecache.ResponseCache.cachekey    -- the computed cache key -->