The web service used in this tutorial is a simple temperature conversion service, provided by the w3schools.com. It provides a Web Services Definition Language document, also called a WSDL, that defines the location of the service, its methods (or functions) and the request and response documents for those methods. Navigate to https://www.w3schools.com/xml/tempconvert.asmx?WSDL to view it.
A WSDL is a very complex document and not readily decipherable, and so the tutorial uses SoapUI to alleviate the need to manually read the file. You can feed this WSDL into a SoapUI project to generate correctly formatted SOAP requests. Once a request has been generated, you can use it to create XSLT style sheets, which will be used with XML Extensions.