So, imagine you're playing a game of tag with your friends. You know how sometimes you might try to tag someone but they run really fast and you can't catch them? Well, imagine you had something that could help you catch them even if they're really far away and running really fast.
That's kind of like what a man-portable air-defense system, or MANPADS for short, does. It's a weapon that soldiers or other people can carry with them, like a backpack or a big tube. When they see an enemy airplane in the sky, they can use the MANPADS to shoot a missile at it and try to bring it down.
The missile has a tracking system that can follow the airplane even if it's moving really quickly or trying to dodge the missile. Once the missile hits the airplane, it explodes and damages it so it can't keep flying.
Some people think that these kinds of weapons are dangerous because they can be used by bad guys to shoot down innocent airplanes, like passenger planes full of people going on vacation. That's why countries try to limit who can have MANPADS and how they can be used.