Imagine you're at home and your friend wants to come over to play. However, they live really far away and don't have a car. What do they do? They might use something called a "carrier" to deliver things to your house.
Now, imagine there are people who work on really big floating houses called "aircraft carriers." These people might need things delivered to them while they're out on the water, and that's where a "carrier onboard delivery" comes in.
A carrier onboard delivery is when a special plane takes off from land and flies out to the aircraft carrier with things like mail, food, and supplies. It lands on a special area of the ship called the "flight deck" and the people on the ship unload the things and bring them inside. Then the plane takes off again and goes back to land.
It's kind of like a special delivery service just for airplanes and ships. Without carrier onboard delivery, the people on the aircraft carrier might not have everything they need while they're out on the water.