A firewall is a security system for your website. It works by blocking access to the sensitive areas from anyone who does not meet the security requirements. Those who don't meet the security requirements will only see the Sucuri page. This makes sensitive parts of your site "invisible" to malicious users... you can't hack what you can't see.

You will know you are being blocked by your firewall when you try to login to your website and you see a Sucuri page instead.