A software firewall is a program installed on a computer or device that monitors and controls incoming and outgoing network traffic. It acts as a barrier between the device and potential threats from the internet, allowing only safe data to pass through while blocking harmful connections. These firewalls can be configured to set specific rules for different applications and services, providing users with customizable security options. By filtering traffic based on predefined criteria, a software firewall helps protect against unauthorized access, malware, and other cyber threats, ensuring a safer online experience.