Code access security is like playing with toys. Some toys are safe and can be played with anytime, but others might be dangerous and can only be played with when grownups say it's okay. The same way, when a computer program wants to do something, there are some things it can do all the time and other things it can only do if it has permission from the grownups (i.e. the people who made the computer system).
This permission system is called code access security. Every computer program has a set of rules and permissions that tell it what things it can do and what things it can't. These rules are used to keep the computer safe and make sure that only the right programs can do the right things.
For example, a program might need to access your personal information to run properly, but it shouldn't be able to share that information with other programs without your permission. Code access security helps control what information programs can access and what they can do with it, so your personal information stays safe.
Think of code access security like a secret club with a list of members who are allowed in. Just like the club has rules about who can enter and what they can do, code access security has rules about what programs can do and what information they can access. By following these rules, we can make sure that only the right people (in our case, programs) can get in and do the right things.