What a JWT is
A JWT, or JSON Web Token, is a compact, self-contained way to securely carry information between two parties, most often used to prove a user's identity in web and mobile apps. After you log in, a server can hand your app a JWT, and your app includes it with later requests to show it is really you. The token holds claims, pieces of information such as your user identity and what you are allowed to do.
The three parts
A JWT has three parts separated by dots: a header, a payload, and a signature. The header states the token type and the signing method. The payload holds the claims, such as the user identity and an expiry time. The signature is the crucial security piece, created by the server using a secret key. Together these parts form a single string that can travel easily in a web request.
How the signature protects it
The signature is what makes a JWT trustworthy. The server generates it from the header, the payload, and a secret key only it knows. When the token comes back, the server recreates the signature and checks that it matches. If anyone altered the payload, say to grant themselves extra permissions, the signature would no longer match and the token would be rejected. This lets the server trust the token without storing it.
Why it enables stateless auth
Because a valid JWT proves its own authenticity through the signature, the server does not need to remember who is logged in. All the needed information travels inside the token itself, so any server that knows the secret key can verify it. This stateless approach makes JWTs popular for systems spread across many servers, where keeping a shared record of every logged-in session would be cumbersome.
Important cautions
JWTs come with caveats worth knowing. The payload is only encoded, not encrypted, so anyone can read it; never put secrets like passwords inside. Because they are hard to cancel before they expire, tokens should be given short lifetimes, with a separate refresh mechanism for longer sessions. Storing them safely in the browser and validating them carefully on the server are essential to avoid common security mistakes.
The bottom line
A JWT is a compact, signed token that carries claims about a user, letting servers verify identity without storing session state. Its header, payload, and signature make it self-contained and tamper-evident, which is why it underpins so much modern authentication. Understanding that the payload is readable and tokens should be short-lived is key to using JWTs securely.
Related on Skillo
See also: What is OAuth? Sign in with Google explained, What is an API? Explained simply.
Sources
Published date reflects the original event date (2026-10-23). This article is original Skillo editorial written from the sources above; facts were verified in September 2026.
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