IDL is an interactive data evaluation environment developed by Research Systems - it is an interpreted language for data manipulation and visualization. The NeXus IDL bindings allow access to the NeXus API from within IDL - they are installed when NeXus is compiled from source after being configured with the following options:
configure \
--with-idlroot=/path/to/idl/installation \
--with-idldlm=/path/to/install/dlm/files/to
For further details see the README (http://htmlpreview.github.com/?https://github.com/nexusformat/code/blob/master/bindings/idl/README.html) for the NeXus IDL binding.