What are JWT claims

The JWT Claims Set represents a JSON object whose members are the claims conveyed by the JWT.  The Claim Names within a JWT Claims Set MUST be unique; JWT parsers MUST either reject JWTs with duplicate Claim Names or use a JSON parser that returns only the lexically last duplicate member name

The following Claim Names are registered in the IANA "JSON Web Token   Claims" registry established by Section 10.1. 

4.1.1.  "iss" (Issuer) Claim

The "iss" (issuer) claim identifies the principal that issued the

   JWT.  The processing of this claim is generally application specific.

   The "iss" value is a case-sensitive string containing a StringOrURI

   value.  Use of this claim is OPTIONAL.

4.1.2.  "sub" (Subject) Claim

The "sub" (subject) claim identifies the principal that is the

   subject of the JWT.  The claims in a JWT are normally statements

   about the subject.  The subject value MUST either be scoped to be

   locally unique in the context of the issuer or be globally unique.

   The processing of this claim is generally application specific.  The

   "sub" value is a case-sensitive string containing a StringOrURI

   value.  Use of this claim is OPTIONAL.

4.1.3.  "aud" (Audience) Claim

The "aud" (audience) claim identifies the recipients that the JWT is

   intended for.  Each principal intended to process the JWT MUST

   identify itself with a value in the audience claim.  If the principal

   processing the claim does not identify itself with a value in the

   "aud" claim when this claim is present, then the JWT MUST be

   rejected.  In the general case, the "aud" value is an array of case-

   sensitive strings, each containing a StringOrURI value.  In the

   special case when the JWT has one audience, the "aud" value MAY be a

   single case-sensitive string containing a StringOrURI value.  The

   interpretation of audience values is generally application specific.

   Use of this claim is OPTIONAL.

4.1.4.  "exp" (Expiration Time) Claim

The "exp" (expiration time) claim identifies the expiration time on

   or after which the JWT MUST NOT be accepted for processing.  The

   processing of the "exp" claim requires that the current date/time

   MUST be before the expiration date/time listed in the "exp" claim.

   Implementers MAY provide for some small leeway, usually no more than

   a few minutes, to account for clock skew.  Its value MUST be a number

   containing a NumericDate value.  Use of this claim is OPTIONAL.

4.1.5.  "nbf" (Not Before) Claim

The "nbf" (not before) claim identifies the time before which the JWT

   MUST NOT be accepted for processing.  The processing of the "nbf"

   claim requires that the current date/time MUST be after or equal to

   the not-before date/time listed in the "nbf" claim.  Implementers MAY

   provide for some small leeway, usually no more than a few minutes, to

   account for clock skew.  Its value MUST be a number containing a

   NumericDate value.  Use of this claim is OPTIONAL.

4.1.6.  "iat" (Issued At) Claim

 The "iat" (issued at) claim identifies the time at which the JWT was

   issued.  This claim can be used to determine the age of the JWT.  Its

   value MUST be a number containing a NumericDate value.  Use of this

   claim is OPTIONAL.

4.1.7.  "jti" (JWT ID) Claim

The "jti" (JWT ID) claim provides a unique identifier for the JWT.

   The identifier value MUST be assigned in a manner that ensures that

   there is a negligible probability that the same value will be

   accidentally assigned to a different data object; if the application

   uses multiple issuers, collisions MUST be prevented among values

   produced by different issuers as well.  The "jti" claim can be used

   to prevent the JWT from being replayed.  The "jti" value is a case-

   sensitive string.  Use of this claim is OPTIONAL.

references:

https://www.rfc-editor.org/rfc/rfc7519#section-4