Hi Sandip, An appservice is a Mendix wrapped webservice. It helps you speedup development to integrate Mendix applications on sharing information. Under the hood an Appservice is just a webservice using an WSDL.
If you download the MSD file from the modeler you can just open it because it's ZIP file with a Mendix extension. I always use 7Zip, but each other tools works just great. The MSD contains an WSDL which can be imported in SOAPUI and then call each operation.