This import option is linked with the export resources option in the way that the import option enables you to import all or some of the resources included in an export.
The export comes as a zip file and you should not unzip the file first, but simply refer to the zip file in the import.
To start the export you need to be signed on to a tenant and here you first need to select Settings tab in the top:
Then you need to select the Import resources in the bottom left:
Now you see this:
Here you first click the icon Upload file and then you see this pop up window:
Here you click Choose file, then you select the zip file and finally click Upload.
Now the screen shows the resources (and resource types) found in the export file and now you can pick and choose the resources, that you want to import:
And when you click Import the resources are copied into the tenant. Please notice, that existing resources with the same name are overwritten without a warning.