This refers to the tension of the film on the core.
Material for archival storage must not be too loose or too tight. Film that has come off a printer or a telecine will be extremely tight or rock solid, which is far too tight for long term storage and must be rewound, as the film is unable to ‘breathe’.
Material that is floppy and risks the core falling out is far too loose, as the wind is supposed to prevent curl. Only a firm wind will do this. This wind is best learned through a practical demonstration.