JSON Web Tokens (JWTs) are like secret message notes that you can pass between two people who trust each other, like you and your best friend.
Imagine you and your friend want to share secrets. You create a note with a couple of fun facts or secrets and fold it up. You write on the outside of the note "to my best friend" and hand it to them.
Now let's translate that into the world of computers. A JWT is like that folded-up note, except it's electronic. Instead of secrets, it contains information about you, like your username or email. Instead of physically handing it to your friend, it's sent over the internet to a server, along with a password.
The server can then use the JWT to authenticate you, meaning it checks to make sure you are who you say you are. If the server verifies that you are who you claim to be, it can give you permission to access certain things.
In short, JWTs are little packets of information that allow you to prove your identity online so that you can access certain things.