This is an editor for .NRO files to modify their icons and other metadata. The info is appended to the end of the .nro file according to the Assets section described on switchbrew.
It will try to leave as much of the original asset data in tact if a .nro is provided that has existing asset information. If there is no asset information (such as libtransistor-built .nro files) it will create the Assets section.
If the pre-compiled binaries don't work, you can also run the python script directly by downloading the source code zip.