Off the Grid pertains to a lifestyle characterized by the lack of a paper trail (ie. no SS number, driver's license, etc.), or a fake identity. People living off the grid may take jobs that pay under the table, move from place to place, and take on numerous identities.
In Terminator 3: Rise of the Machines, John Connor was living off the grid in order to avoid detection by agents of Skynet. The fact that he inadvertently ran into Kate Brewster was probably the only reason that the T-X and T-850 found him. Before her death (in the T3 timeline, Sarah Connor was probably living off the grid as well.