Fuyusoden refers to rice fields that were exempt from taxation under the ritsuryō system.
輸租田 were taxable rice fields under the same ritsuryō system, in contrast to the tax-exempt 不輸租田.
Compound of 不 (fu, 'not'), 輸 (yu, 'transport' or 'pay'), 租 (so, 'tax'), and 田 (den, 'rice field'). The term is a technical legal designation from the ritsuryō period.