A cartridge trap is kind of like a very special container that is made to catch and hold things. It looks kind of like a little box with a lid that can open and close.
The way it works is you put something inside the box, and then when something else comes along and tries to get at it, the box closes up and traps the other thing inside.
People use cartridge traps for all kinds of things. Sometimes they use them to catch animals that are causing problems, like if a pesky raccoon keeps getting into the garden and eating all the vegetables. Other times they use them to catch bugs or insects that they want to study or get rid of.
Cartridge traps are really special because they are very precise and can be set up to catch specific kinds of things. For example, if you wanted to catch only flies and not other bugs, you could set up a cartridge trap with special bait that only attracts flies.
Overall, cartridge traps are a really cool tool that people use when they need to catch and hold things in a very specific way.