HTTP Body Data 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).
Most HTTP requests are GET requests without bodies. However, simulating requests with bodies is important to properly stress the proxy code and to test various hooks working with such requests. Most HTTP requests with bodies use POST or PUT request method.
The message body part is optional for an HTTP message but if it is available then it is used to carry the entity-body associated with the request or response. If entity body is associated then usually Content-Type and Content-Length headers lines specify the nature of the body associated.
A message body is the one which carries actual HTTP request data (including form data and uploaded etc.) and HTTP response data from the server ( including files, images etc). Following is a simple content of a message body:
<html> <body> <h1>Hello, World!</h1> </body> </html>
For more details to HTTP messages and bodies refer to w3org link