What does silence at the reaping mean in the book hunger games?

There was silence at the reaping first of all because Katniss volunteered as tribute for her little sister......no one has volunteered as tribute in a while at District 12. Also there was kind of some relief that Prim (little sister) didnt go because there were a good number of people that knew her, they also knew Katniss but being that Prim was younger and knew more people.