License codes are usually used if you are selling softwares and want to give your customers unique code for each purchase so that they can use it to activate his software.
Basically, if you use this feature, for each document, you will need to create different license codes. When users order one document, the extension will get an available license code and send to the customer. That customer then can use that license code to activate the software his purchase.
Each license code can only be used one time. And usually, for normal usage, you don't need to use this feature.