A demilitarized zone is an area of land that has no military forces in it. This is usually to stop armies from fighting.
The most famous demilitarized zone is the Korean Demilitarized Zone on the land between North Korea and South Korea. No one can go into the Demilitarized Zone, so the wildlife is well preserved. There are thousands of soldiers and weapons on each side of the Demilitarized Zone, including to this day a massive American presence.