The Machine Id is a unique identifier assigned to a computer when you publish it, or add it, to NoMachine Network.
It works like an IP address allowing NoMachine users with a NoMachine Network subscription to connect to that machine without knowing its IP address. The Machine Id is generated when you go to Settings > Network > Machine > Add in NoMachine. 

Instead of connecting via a numerical IP address in the NoMachine player, you connect to a machine using a unique Id generated by NoMachine. Users must first login on the player with their NoMachine User Id, and then create the remote desktop connection using the target computer's Machine Id. The NoMachine Network services allows connections to computers located behind routers (NAT) or firewalls without needing port forwarding.