A firewall in the context of computer security is a network security device or software that monitors and controls incoming and outgoing network traffic based on predetermined security rules. It acts as a barrier between a trusted internal network and untrusted external networks, such as the internet. Firewalls are designed to prevent unauthorized access, protect against malicious activities, and manage network security by inspecting and controlling data packets based on specified criteria, such as source and destination IP addresses, ports, and protocols.