What does the Browser Cache Expiration do?
The Browser Cache Expiration specifies how long cached files will remain in your visitor's browser cache. This expiration time is what CloudFlare will set unless longer time periods are specified at your web server.
A longer expiration time ensures faster load times for repeat visitors. However, a longer expiration time also means slower update times if those files are modified.
By default CloudFlare caches static resources for 4 hours.
To learn more about Browser Cache Expiration see here.
Note: You can also increase the cache expiration by specifying a longer cache time on the origin server, or you can set a different Cache Expiration for a specific path or resource using the Page Rules app.