The term off the grid means any planet that can sustain Human life, yet has no Stargate. The term was first used by the Atlantis Expedition. They are usually like this because either the Ancients deemed it too small (ie habitable moons) or too lacking in resources to be worth building a gate for. Alternatively the gate may have been destroyed or buried, or in Lantea's case, moved. This is why most planets in the network contain Naquadah, as it is a key ingredient in most Ancient technology and most planets containing it would have been first choices for the network.