A Revenue Farm is a franchise with a license to collect state revenue, with a monopoly

Monopoly

A monopoly exists when a specific person or enterprise is the only supplier of a particular commodity...

right to practice a certain business. Such a monopoly was granted by the state government to a "Farmer" for a limited period of time in a strictly defined location in a city, district or province. In return, the farmers/bidders had to pay the government in advanced to be guaranteed monopoly status.