If you live off-grid, your home is not plugged into the usual public supplies — the electricity wires, the gas pipes, the water pipes that most towns run. People off-grid often make their own power with the sun, and they may collect rainwater. Some choose it on purpose, for a quieter or greener life; some live that way because the wires never reached them. You also hear it for a cabin or a van, not only a house. It is fairly new, ordinary English in news and lifestyle talk. Grid here is that public net of supplies, not a pattern of squares. Though actually people also stretch it to mean dropping off the phone and the internet, as if they had disappeared — that is looser talk, not the core meaning.