Payload it the data recevied or set in the flow. If flow is hit by some external source, the submitted data found in the payload and you can set the payload manually in the flow as well.
The type of the payload depends on the source that is generating the payload. For example, if you use the JSON to Object Transformer, the payload type would be the type of the class you specified as return class of the component Josn to Object transformer. The payload could be HashMap object in which you retrives the data by using the key in the get method.
You can convert the payload as per your requirement.
<flow name="Form_submit">
<http:listener config-ref="HTTP_Listener_Configuration" path="/form" allowedMethods="POST" doc:name="HTTP"/>
<object-to-string-transformer doc:name="Object to String"/>
<logger message="User #[payload]" level="INFO" doc:name="Logger"/>
<set-payload value="Payload Received is: #[payload]" doc:name="Set Payload"/>
</flow>
