Bare root is, as the name suggests, with bare roots. So they ship plants with the roots in a cloth, plastic etc; Anything to keep them moist for a couple of days. Removing the soil makes transport a lot easier (And with the competition you need bare root anyway). Typically these are 103 year old plants used for larg scale planting work (Think hedgerows, municipal grounds, forests). It saves a lot on transportation cost & effort. And as you order them in winter, the successrate withplanting this material is normally similar to that of 'plants in pots'