Adding "What is JWT"

Author: BaseMax

README.md

@@ -16,13 +16,20 @@ composer require firebase/php-jwt
 - Run Encode: `$ php encode.php`
 - Run Decode: `$ php decode.php`
 
+### What's JWT?
+
+**JSON Web Tokens** are an open, industry standard RFC 7519 method for representing claims securely between two parties.
+
+JSON Web Token (JWT) is a compact, URL-safe means of representing claims to be transferred between two parties.
+
+The claims in a JWT are encoded as a JSON object that is used as the payload of a JSON Web Signature (JWS) structure or as the plaintext of a JSON Web Encryption (JWE) structure, enabling the claims to be digitally signed or integrity protected with a Message Authentication Code (MAC) and/or encrypted.
 
 ### Related pages
 
 - https://jwt.io/
 - https://github.com/firebase/php-jwt
 - https://github.com/cdoco/php-jwt
-
+- https://tools.ietf.org/html/rfc7519
 
 ### Acknowledgment