IDL, short for Interactive Data Language, is a programming language used for data analysis.
There two types of IDL licenses available:
The IDL User Floating License can be used to create SAVE (.sav) files of IDL Program Files (.pro).
The IDL RunTime Cluster License is used to run the SAVE (.sav) files in multi-node environment such as the Developer Cloud Sandbox.
To learn more about IDL:
Wikipedia Article about IDL
Exelis Visual Information Solutions IDL home page