Okay kiddo, so you know how when you learn a new word or hear a new phrase, you might not know exactly what it means? Well, it's the same when people in the military talk to each other. They use special words and terms that can be hard for other people to understand.
That's where the Department of Defense (DOD) Dictionary of Military and Associated Terms comes in. It's like a big book of all the special words and phrases that people in the military use, and what they mean.
For example, let's say someone in the army says the phrase "MOPP level 4". That might sound really confusing to someone who doesn't know what it means. But if they look it up in the DOD Dictionary, they'll see that it stands for "Mission-Oriented Protective Posture level four" which is just a fancy way of saying that they're wearing special clothes and equipment to protect them from bad stuff like chemicals or germs.
So basically, the Department of Defense Dictionary of Military and Associated Terms is like a big book of special words and phrases that helps everyone in the military speak the same language and understand each other better.