The main purpose of a firewall in network security is to monitor and control incoming and outgoing traffic based on predetermined security rules. Firewalls act as a barrier between a trusted network and untrusted networks, such as the internet. They help prevent unauthorized access, protect sensitive data, and mitigate threats like malware and hacking attempts. Firewalls can be implemented through hardware or software solutions, and configuring them correctly is vital for maintaining robust security and safeguarding organizational resources and information.