A JSON Web Token, or JWT, is a small signed string that a server gives a user as proof of identity. After someone logs in, the server creates a token containing details like the user ID and what they are allowed to do, signs it with a secret key, and hands it over. The user sends that token back on every request, and the server only has to check the signature.
Picture a festival wristband. At the gate you show your ticket once, get a band, and from then on the band itself is enough to get you in and out without queuing again. A JWT works the same way: the signature is the tamper-proof seal that the holder is genuine, so the server does not need to re-check the database each time. This is the backbone of modern token-based authentication and a common building block of OAuth flows.
A JWT comes in three parts: a header saying how it was signed, a payload holding the claims, and the signature itself. The first two are just encoded text, not encryption, which is why anyone can read them. That readable payload is handy for carrying basic facts about a user, but it is also exactly why authentication systems keep it free of anything sensitive.
One catch worth knowing: a JWT is signed, not hidden. Its payload is readable by anyone, so it should carry claims, not secrets. And because anyone holding the token can use it, tokens are kept short-lived and only ever sent over an encrypted connection.
At TopDevs we use JWTs to keep APIs stateless and fast, while pairing them with short expiry and revocation so a leaked token cannot be abused for long.
