This setting is how CloudFlare communicates with the visitor's browser. CloudFlare will respect the headers that you send to us from your web server if the origin tells us to not cache the resource or if the TTL sent from the origin is greater than the TTL specified in the browser cache expire TTL.
For example, your origin server headers may be set to cache for 1 week. CloudFlare will respect these headers so long as the browser cache expire TTL set in CloudFlare is shorter than 1 week. If you set the browser cache expire TTL to a time period greater than one week, such as 1 month, CloudFlare will overwrite the cache header set by the origin.