What is Payload?

SOAP Payload is a SOAP transaction envelope 

To execute a SOAP method call, the SOAP payload must be constructed with a root element containing an optional element and a mandatory element.

The element is used to encapsulate extended information about the call. The element contains the actual method call (as its first child element) and the associated method parameters as either embedded or independent elements.

string in valid json format { "pushToken":"ABCDEFG" } geting payload var pushtoken = JObject.parse(json)["pushToken"];

It is just the Input we pass to the rest API and also the output we receive from the rest API. Generally these input/outputs are in json format.