These two characters are Nora and Krogstad. I think what's happening in this scene is when Krogstad comes to visit Nora about how she forged her father's signature. At the time Nora told her old friend Mrs. Linde that she raised the money for her once sick husband to go away when Mrs. Linde thought Nora burrowed the money from her father.
Krogstad knows how this is a big deal because he once did the some thing back in the day. If Nora doesn't pay up Krogstad will tell her husband about what she did or start blackmailing her. According to Nora its not a big deal. A quote from Act I is " A barrister's life is such an uncertain one- especially when he won't touch any case that isn't absolutely respectable" (page 155)