SPDisco.aspx file is a directory of all web services available in SharePoint.

This file contains names and locations of all web service endpoints which is available in SharePoint.
If you are creating  custom web service in SharePoint then you must do an entry of endpoints in SPDisco.aspx file.

Location of SPDsico.aspx file is:
“C:Program FilesCommon FilesMicrosoft SharedWeb Server Extensions14ISAPI”