This is, literally, a house in which things grow, but what is grown in a grow house is the rather special plant called marijuana. The entire house is given over to cultivating these plants hydroponically, which is effective horticulturally and means they can’t be seen from outside. There are various ways of disguising the house, as by setting up lights on a timer between the windows and the blinds so that neighbours think that someone is living there. Naturally the windows are all covered to conceal the strong lights inside required by the plants. Since lights and ventilation require a lot of electricity, the power supply is diverted from the mains so that the usage cannot be tracked to a particular house.
The term is an Americanism, but, since the illegal cultivation of marijuana is an international operation, the associated jargon has travelled to our shores.