Let's assume you are about to enter a site with adult content (don't worry, you aren't, it's only a demo!). Click over 18 to dismiss this message, or 18- to leave this site.
Since you see this page, you must have clicked over 18!
A session cookie named DoorKeeperDemo1 has been set, so you will not see the message again this browser session.
On a ‘real’ site, I suggest you set the cookie to expire after a long time, so your visitor will not be bothered with the message again.
Want to check? Go back to the DoorKeeper stack product page and click the link for demo#1 and the message will not show again.
Remove the cookie set by this page now