What is a "node license"?

Node license (often shortened to "N") is a license that may be used on a node PC only - where the host application (3ds Max, C4D, ...) is running in command line mode, with no GUI - for example when rendering using Corona's Distributed Rendering in 3ds Max and/or Autodesk Backburner, or using Team Render in Cinema 4D. It is the opposite of workstation license.

Each workstation license can be also used as a render node license, so having 2 x [1 workstation + 3 render nodes] licenses means that you can use them on 2 workstations and 6 render nodes, but also on 1 workstation and 7 render nodes.

Note: you do not need a 3ds Max license to render on a node PC. Trial license of 3ds Max can be used for this, even if it's already expired. This is absolutely fine according to the Autodesk licensing: 

The number of licenses needed depends solely on the number of workstations that you would like to use to do actual 3ds Max work on. For example, if you only have one artist who works with 3ds Max/3ds Max Design, then you only need one license. All other render node computers must have 3ds Max installed but not necessarily activated.

Source: [link]