HTTP headers let the client and the server pass additional information with a message in a request or response.
In HTTP/1.X, a header is a case-insensitive name followed by a colon, then optional whitespace which will be ignored, and finally by its value (for example: Allow: POST).
In HTTP/2 and above, headers are displayed in lowercase when viewed in developer tools (accept: */*), and prefixed with a colon for a special group of pseudo-headers (:status: 200).
You can find more information on the syntax in each protocol version in the HTTP messages page....If-Unmodified-Since use this value to change the behavior of the request...If-None-Match use this value to change the behavior of the request...