Mirror writing is when someone writes backwards, as if they are looking in a mirror. So instead of writing "hello" as "hello", they would write it as "olleh". It's kind of like a secret code that only someone who can read backwards can understand. You might see mirror writing used in things like secret diaries or codes that only certain people can understand. Some people naturally write this way, but others have to learn how to do it on purpose. It's a fun way to communicate secretly with friends, but it can be really tricky to read if you're not used to it!