A De-Militarized Zone (DMZ) is a feature that allows a device connected to the WiFi router to have direct access to and from the internet.
This means that any device placed into the DMZ is not subject to any of the security features and precautions offered by the WiFi router, such as port forwarding, network address translation (NAT), firewalling, or more. Hackers have direct access to the device, as it is no longer protected by the WiFi router, meaning the device must protect itself.
A common practice is for gamers, for example, to put their gaming machines into the DMZ so they can bypass firewall rules that limit their access to online games. Unfortunately, most gaming machines (and other devices) are not sufficiently protected on their own to prevent hackers from compromising them.
While just having a DMZ enabled is not bad by itself, it is a very dangerous place to put any device that is not sufficiently protecting itself. Even the user guide for the lab's test WiFi router stated “Note: Placing a computer in the DMZ may expose that computer to a variety of security risks. Use of this option is only recommended as a last resort.”