HTTP headers - HTTP | MDN

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.