Syntax

import com.sap.gateway.ip.core.customdev.util.Message;

def Message processData(Message message) {

// Your code goes here 

return message;

} 

Explanation:

import com.sap.gateway.ip.core.customdev.util.Message library which is must have to be there in Groovy file.  

Message

One class which handle message that are passed through the iFlow. The Message contains the header, properties and the message body.

def Message processData(Message message) {

This line defines a method/function called processData

It accepts one input parameter named message which type Message.

The return type is also Message(which is actual message not the parameter)