A JSON Web Token is three Base64url-encoded segments separated by dots: a header saying how it was signed, a payload of claims, and a signature. The first two are just encoded, not encrypted, so anyone holding the token can read them. This tool decodes those segments and presents the claims in a readable form.
In practice you use it while debugging authentication. A request is coming back 401 and you want to know whether the token has expired, which subject it is actually for, what audience it was issued to, or whether the scopes and roles you expected are really in there. Reading that off the raw token by eye is not feasible, and pasting a production token into an unknown website is a bad habit.
Registered claims get special treatment. The three timestamp claims, exp, iat and nbf, are stored as Unix seconds, which tell you nothing at a glance; they are shown here as absolute dates alongside a relative description, plus an explicit expired or valid indicator based on your own clock. That single check answers the most common question about a token.
One thing this tool deliberately does not do is verify the signature. Verification requires the issuer's secret or public key, and a browser tool that asked you to paste your signing secret would be teaching a genuinely dangerous habit. Decoding tells you what a token claims; only your server, holding the key, can tell you whether to believe it.