The individual lines are called “header fields”. Each (except the first) consists of a name/value pair separated by a colon.
Key to the individual symbols:
- HTTP/1.1 is the valid HTTP protocol version.
- 200 OK is the Status-Code. It says that the server has received, understood and accepted the request.
- Content-Encoding and Content-Type tell us about the type of file.
- Age, Cache-Control, Expires, Vary and X-Cache refer to the caching of the file.
- Etag and Last-Modified are used for version control of the delivered file.
- Server refers to the web server software.
- Content-Length is the file size in bytes.
As you can see, this header information is mainly used for coordination between the client (browser) and the server. It is ensured that the client can understand the form of the file, that the file is sufficiently up-to-date and that the file size meets the browser's expectations.
The header lines shown in the example are only a small part of the available header fields. In total, there are almost 100 HTTP header fields, of which about 30 are for specifying HTTP requests, about 30 for the server response, and a whole range of other header fields that serve different purposes and are partly (still) not standardized.
The following overview explains the most important header fields.