"I'm Under Attack" will add additional protections to a site that is under attack and needs DDoS protection and mitigation. If you're under attack and have this feature enabled during the attack, visitors will receive an interstitial page for about five seconds while we analyze the traffic to make sure it is a legitimate human visitor.
To enable I'm Under Attack mode, see How do I enable I'm Under Attack mode.
Why am I getting a "Checking your Browser before accessing" message before entering a site on CloudFlare?
The "Checking your browser before accessing example.com" page occurs when the site owner has turned on this I'm Under Attack Mode setting. The page will generally go away and grant you access to the site after 5 seconds.