Okay kiddo, so you know how sometimes people want to find out secrets from other countries or groups of people? Well, there are some people who are hired by their own country to go and live in that other place and try to find out those secrets. They are called resident spies.
Imagine you are playing a game of hide-and-seek with your friends. One of your friends goes and hides in a hard-to-find spot and doesn't come out for a while. That friend is kind of like a resident spy. They are hiding in another country or group and trying to find out secret information, but they are doing it in a sneaky and hidden way.
Sometimes, resident spies have really cool gadgets or tricks to help them find out the secrets they are looking for. But they have to be very careful not to get caught, because if they do, they could get in big trouble with the people they are spying on.
So basically, a resident spy is someone who goes and lives in another place to find out secret information, and they have to be very sneaky and careful so they don't get caught.