Let's say the browser's cache expires. The browser will attempt to grab the latest copy of the page and CloudFlare will immediately return the copy in its own cache unless its own cache is expired, in which case CloudFlare will grab the latest copy of the page from your origin server.
If you think about this behavior you'd know this is to your advantage. Client browser has a small TTL which means it almost always grabs fresh content from CloudFlare, and CloudFlare will not disturb your origin server unless its cache is expired.
This essentially means you are putting the workload on the CDN which is what a CDN is good for.