MASINT, or measurement and signature intelligence, is how we learn about things we can't see by measuring the things we can see. Say you're in a dark room and you hear someone laughing. You can't see them, but you know they're there because of the sound. Similarly, in masint, we measure certain things to learn about other things we can't see directly.
For example, we can't see underground tunnels or caves, but if we measure the temperature and humidity near the ground, we can tell if there's a space beneath us. We can also learn about what kind of vehicles are moving around by measuring the heat they give off or the radar waves they reflect.
It's kind of like being a detective and looking for clues to figure out what's going on. MASINT helps us learn about things in a more sneaky way so we can stay safe and protect ourselves and others.