Hi,
What the following statement "Each resource can only exist in one resource group." means is that you can only have one (1) unique named resource in each resource group.
For example:
The Azure virtual machine "VM-01" can only exist once in the resource group "RG-01", if you attempt to move your other "VM-01" which resides in the resource group "RG-02" to the resource group "RG-01" it will say "Virtual machine name must be unique in the current resource group."
Or if you attempt to create new Azure virtual machine with the name "VM-01" to the resource group "RG-01" it won't work.
Best regards,
Leon