A 6 digit key that (when loaded) contains information about the Ignition Gateway and it's modules. You can run an unlicensed Ignition in Trial mode, or you can load a license and have unlimited access to the modules. You can also install modules that are not on the key, and they will run in trial mode.
In Ignition we refer to libraries in several places, but it just means a collection of similar items. These could be a collection of images, scripting functions, components, or more.