HTTP Message Body is the data bytes transmitted in an HTTP transaction message immediately following the headers if there are any (in the case of HTTP/0.9 no headers are transmitted).
The request/response message consists of the following:
GET /logo.gif HTTP/1.1
or Status line, such as HTTP/1.1 200 OK
,The request/status line and headers must all end with (that is, a carriage return followed by a line feed). The empty line must consist of only and no other whitespace.
The "optional HTTP message body data" is what this article defines.
This could be a response from the web server:
Hello world!
The message body (or content) in this example is the text Hello world!.