The main purpose of a firewall in computers is to protect networks and systems from unauthorized access and cyber threats. Firewalls act as a barrier between trusted internal networks and untrusted external networks, monitoring and controlling incoming and outgoing traffic based on security rules. They can be hardware-based, software-based, or a combination of both and are essential for safeguarding sensitive data and maintaining the integrity of systems. By filtering traffic, firewalls help prevent malware, hacking attempts, and data breaches, ensuring a secure computing environment for users and organizations.