Okay, so you know how sometimes people say mean things to each other? Well, hate mail is when someone writes those mean things down and sends them to someone else.
It's like when you draw a picture for your friend and you give it to them, except instead of a nice picture, it's a mean letter that makes the other person feel sad or angry.
Hate mail can come in different forms, like letters, emails or messages on social media. It can be from someone you know or from a stranger. It's not a nice thing to do and it can hurt people's feelings.
It's important to remember to always be kind and respectful to everyone, even if we don't agree with them or we don't like them.