GNU Mail is a special kind of computer program that helps you read and send email messages. It is part of a big collection of free and open source software called the GNU Project, which is like a big toy box full of fun and useful computer tools that people can use and share with each other.
When you use GNU Mail, it's like having a special mailbox on your computer where all your email letters are stored. You can open up your mailbox and see all of the letters people have sent you. If you want to write a letter to someone else, you can use GNU Mail to compose a new message and send it to them.
One of the cool things about GNU Mail is that it works with lots of different email services, like Gmail and Yahoo Mail, so you can use it to manage all of your email in one place. It also has lots of different features, like the ability to organize your messages into folders, search for specific messages, and even encrypt your messages so that only the person you're sending them to can read them.
In short, GNU Mail is like a magical mailbox on your computer that helps you keep track of all your emails and send new ones to your friends and family.