Okay kiddo, so you know how sometimes big ships, like boats or airplanes, need to protect themselves from bad guys who might try to attack them? Well, a close-in weapon system is like a special kind of gun that sits on the boat or plane and shoots at the bad guys to keep them away.
It's called a "close-in" weapon system because it's meant to shoot at things that are very close to the ship or plane - like if the bad guys are flying towards the ship in a small airplane, or if they're in a boat nearby. The close-in weapon system can shoot very fast and accurately, so it can take down the bad guys before they can harm the ship or plane.
The system is controlled by specially trained people on the ship or plane, who watch for any incoming threats and tell the close-in weapon system where to aim and shoot. The system is designed to be very precise, so it doesn't accidentally shoot at things that aren't a threat.
Overall, a close-in weapon system is like a big protector for ships and planes, shooting at bad guys to keep everyone safe.