Imagine you have a safe with a lock that has a special code for opening it. Only the person who knows the code can open the safe.
RSA numbers are like complex codes that are used to secure messages or transactions on the internet.
The RSA code is made up of two big numbers, let's call them A and B. A and B are really, really big numbers with lots of digits. They are so big that it would take a really, really long time to add them together or to multiply them.
Because A and B are so big, it is nearly impossible for someone to guess what they are just by randomly guessing numbers. This makes RSA codes very secure for protecting information like bank transactions or email messages.
When you want to send a secret message to someone, you can use A and B to create a code. The person receiving the message has a special key that allows them to unlock the code and read the message.
But don't worry, you don't need to remember all of this because your computer does it for you automatically when you send secure messages online!