A firewall is a device or software that monitors and controls incoming and outgoing network traffic based on predefined rules or policies. A firewall can help prevent unauthorized or malicious traffic from entering or leaving a network, and protect network resources from external threats. Placing a firewall in between the corporate network and the guest network can help prevent a guest device from infecting machines on the corporate network or taking down the company's single internet connection, as it can block or filter any unwanted or harmful traffic from the guest network.