The Item URL in the command you provided refers to the URL of the document library that you want to transfer. You can find the URL of a document library by navigating to it in the SharePoint site, and then copying the URL from the address bar of your web browser. The URL will typically be in the format of
https://<your-site-name>.com/<document-library-name>
For example, if you have a SharePoint site called "mycompany" and a document library called "Marketing Materials", the Item URL for that library would be "**https://mycompany.com/Marketing%20Materials**".
You can also find the URL of the document library by using SharePoint PowerShell. You can use the command:
Get-SPWeb <Web-URL> -Limit All | Select Title, Url
It will show you the title and URL of all Lists/Libraries on the SharePoint web specified in <Web-URL>
You can then use this URL in the Export-SPWeb command to transfer the document library to the specified backup path.