A poison pen letter is when someone writes a nasty, mean or hurtful letter to someone else with the intention of causing them emotional distress. It's like when someone says something mean to you or calls you a bad name, but instead of saying it to your face, they write it down on a piece of paper and send it to you.
Sometimes, people write these types of letters anonymously, which means they don't put their name on the letter. This makes it hard for you to know who wrote it, which can make you feel even worse.
It's important to remember that these letters are not nice, and they can cause a lot of harm to the person who receives them. If you ever receive a poison pen letter, it's best to tell an adult you trust, like a parent, teacher, or friend, and they can help you deal with the situation.