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