When Starkiller Base tests out its destructive power, it wipes out the planets in the Hosnian System, most notably Hosnian Prime. The planet served as the seat of the New Republic, formerly the Galactic Senate, the government of the galaxy formed after the Battle of Endor to restore order to the galaxy and ensure the Empire's forces didn't rise again. Except, well, that's exactly what happened with the First Order. They rose, and the Resistance was put in place to fight them - officially separate from the New Republic, but secretly backed by them. Now, though, that might be about to change. The base of the New Republic has been destroyed, and it's unclear what remains. Even if it does survive, Leia's emissary (seen in deleted scenes) was killed during the strike, meaning ties will be strained if not completely severed. The Resistance was already pretty small, and now in Episode VIII they could well find themselves without any backing, which is going to make their job against a resurgent First Order even more difficult.