A phone farm is a rack or array of real physical smartphones, powered and connected so they can run mobile apps continuously. Agencies and operators use farms to manage many social accounts on authentic devices rather than on easily-detected emulators.
Running your own farm is expensive and fiddly: you have to buy the handsets, supply SIMs, manage power and USB hubs, handle reboots and cable failures, and keep everything online 24/7. It is real infrastructure work.
A cloud phone service is a phone farm you rent by the device instead of building yourself. DistrictDroid runs the hardware — real Pixel phones on US carrier SIMs in Texas — and streams each one to your browser. You get the authenticity of a physical farm with none of the maintenance, and you can scale up or down per rental.