Payload body

The HTTP message payload body is the information ("payload") part of the data that is sent in the HTTP Message Body (if any), prior to transfer encoding being applied. If transfer encoding is not used, the payload body and message body are the same thing!

For example, in this response the message body contains only the payload body: "Mozilla Developer Network":

HTTP/1.1 200 OK
Content-Type: text/plain

Mozilla Developer Network

By contrast, the below response uses transfer encoding to encode the payload body into chunks. The payload body (information) sent is still "Mozilla Developer Network", but the message body includes additional data to separate the chunks:

HTTP/1.1 200 OK
Content-Type: text/plain
Transfer-Encoding: chunked

7\r\n
Mozilla\r\n
9\r\n
Developer\r\n
7\r\n
Network\r\n
0\r\n
\r\n

For more information see RFC 7230, section 3.3: Message Body and RFC 7230, section 3.3.1: Transfer-Encoding.