Firewall is used to block or selectively allow access to services running in system. For example, you have Apache web server and SSH running on your server. You want to allow everyone connect to your Apache Web Server. But only white listed IP address connect to SSH server.

• Managing Firewall Rules on Cent OS 7
• Blocking Layer 7 DDoS with CloudFlare Firewall using Fail2ban
• Enable CSF firewall for WHM Resellers
• open3: exec of ipset flush failed at csf