A demilitarised zone, or DMZ for short, is a special area where people agree not to bring weapons, like guns or tanks. It's kind of like a playground with a fence around it where no one is allowed to bring anything that could hurt someone else.
In the real world, countries sometimes need to agree to have a DMZ along their borders if they don't get along with each other. That way, they can have a safe space where they can talk or trade without worrying about fighting.
For example, there is a DMZ between North Korea and South Korea. This area is really important because these two countries don't get along and have had fights in the past. The DMZ helps keep them from attacking each other and is a place where they can talk and try to work out their problems in a peaceful way.
So, a DMZ is like a special area where people agree not to bring any weapons and can work out their problems in a peaceful way.