PowerShell Direct (PSD) is a new feature with Windows Server 2016 (and on Windows 10 Anniversary Update or later). PSD enables you to use PowerShell remoting to access a Hyper-V VM without needing to setup networking and firewall settings. With PSD, you use Invoke-Command, specifying either the VM's name or the VM's VMID (the VMID is a GUID used internally by Hyper-V to identify a VM). You can also use the VM name or VMID to enter a remote session using Enter-PSSession.
In previous versions of Hyper-V, you needed some sort of networking connection between your Hyper-V host and the guest OS in order to remote into the guest. This was often setting up firewall exceptions and establishing network connectivity. With PSD, you can use the VM's name or ID and remote straight in.