A firewall serves as a security barrier between an internal network and external threats, controlling the incoming and outgoing network traffic. It uses predefined rules to permit or block data packets, protecting data from unauthorized access and malicious attacks. Firewalls can be either hardware-based or software-based, and they play a critical role in ensuring data security by monitoring traffic and preventing potential breaches. In summary, firewalls are essential for maintaining network integrity and safeguarding sensitive information.