Adam,
When you create an image or a "box" using Packer, it will generate the finished artifact using the format of the builder and, depending on the builder you're using, either leave the artifact (box file) on the file system, or register it with, for example, the Azure image inventory. (If you used an Azure builder)
In your case, you're using the "hyperv-iso" builder which will leave a box file in which there is a disk file and metadata that HyperV will understand. However, since you want to run that newly generated artifact/box file with Vagrant, Vagrant first needs to know it exists. It was left on your file system but Vagrant needs to be told about it. So, when you "vagrant box add my-ubuntu <path to output .box from packer build>" per Rickard, that adds the box you generated to the Vagrant inventory.
Then you can reference that box from any Vagrant file on your system and "vagrant up" and it'll use your newly generated box from the Vagrant inventory.