Farming or tax-farming is a technique of financial management in which the management of a variable revenue stream is assigned by legal contract to a third party and the holder of the revenue stream receives fixed periodic rents from the contractor. It is most commonly used in public finance, where governments (the lessors) lease or assign the right to collect and retain the whole of the tax revenue to a private financier (the farmer), who is charged with paying fixed sums (sometimes called "rents", but with a different meaning from the common modern term) into the treasury.
Farming in this sense has nothing to do with agriculture, other than in a metaphorical sense.