A key in cryptography is like a secret password that helps keep your messages and information safe from people who shouldn't have it. Imagine you are sending your friend a message, but you don't want anyone else to know what you're saying. You can use a key to jumble up the words and letters in your message so that it looks like nonsense to anyone who doesn't have the key. When your friend receives the message, they use the key to un-jumble it and read what you said.
It's important to keep the key a secret, just like you wouldn't want anyone to know your password or the secret hiding place for your favorite toy. If someone knows your key, they can un-jumble your messages and read all of your secret information. That's why people who use cryptography to protect their information have to be very careful to keep their keys safe and secure.