In firewall configurations, an 'explicit deny' rule is usually placed at the end of the access control list (ACL) to deny any traffic that does not match any of the preceding permit rules. This acts as a catch-all to prevent any unintended or unspecified traffic from entering the network, ensuring that only traffic that has been explicitly allowed through matching rules is permitted. An explicit deny rule is crucial for maintaining a secure network posture, as it enforces a default state of denying access, thereby protecting against unknown threats or overlooked configurations.