The mode is a major mode to support editing command files of the Interactive Data Language (IDL), a proprietary language owned by Research Systems Inc (www.rsinc.com). The package also contains support for running the idl program as an inferior shell. Please note that this "idl" has nothing to do with CORBA's idl. The mode is called `idlwave-mode' and the associated shell `idlwave-shell-mode'. I have signed the papers with the FSF for these packages and installed the mode in the Emacs source tree recently.