Private networks give you an isolated internal network on which your virtual machines can securely communicate.
Your LunaNode account comes with one private network in each region. Additional networks are $4/mo each.
If you have multiple private networks in a region, you'll be able to select a network when provisioning a virtual machine. You can also modify the private network of an existing VM, or attach an additional network interface to the VM, by selecting the VM and then opening the IP tab.