HTTP headers are the name or value pairs that are displayed in the request and response messages of message headers for Hypertext Transfer Protocol (HTTP). Usually, the header name and the value are separated by a single colon. HTTP headers are an integral part of HTTP requests and responses. In simpler terms, HTTP headers are the code that transfers data between a Web server and a browser. HTTP headers are mainly intended for the communication between the server and client in both directions.
Techopedia explains HTTP Header
HTTP headers can be mainly classified into two types:
HTTP Request Header
Whenever you type a URL into the address bar and try to access it, your browser sends an HTTP request to the server. The HTTP request header contains information in a text-record form, which includes particulars such as the type, capabilities and version of the browser that generates the request, the operating system used by the client, the page that was requested, the various types of outputs accepted by the browser, and so on.
HTTP Response Header
Upon receiving the request header, the Web server will send an HTTP response header back to the client. An HTTP response header includes information in a text-record form that a Web server transmits back to the client's browser. The response header contains particulars such as the type, date and size of the file sent back by the server, as well as information regarding the server.