Imagine you have a secret clubhouse, and you want to make sure only your friends can come inside. So, you create a special secret handshake that only you and your friends know.
When someone comes to the clubhouse door, they must perform the secret handshake. If they do it correctly, you know they are your friend, and you let them in. If they don’t know the secret handshake, they can’t come in.
Authentication is like that secret handshake. It’s a way for computers and websites to make sure that you are who you say you are, usually by asking for a username and password. If you enter the correct information, you can access the website or service, just like entering the clubhouse. If you don’t, you’re not allowed in.