A firewall is a network security device that monitors and controls incoming and outgoing network traffic based on predetermined security rules. It establishes a barrier between a trusted internal network and untrusted external networks, such as the Internet. Firewalls can be hardware-based or software-based. They are essential for network security as they help protect against threats and unauthorized access.

References:

Cisco DevNet Associate Exam Topics: Network Fundamentals (understand the role and function of firewalls in network security)

Cisco Networking Basics: Network Security Devices and Firewalls (explains the functions and importance of firewalls)