Callbacks are a way for the production team to see more from the actors if needed. Getting a callback or not getting a callback does not mean anything good or bad, it just means either the director needs to see more or doesn't need to see more to make her decisions.
If you are asked to come to a callback, please dress comfortably and be prepared to dance! Please remember that there is nothing to be nervous about! Your production team wants you to do well and wants to encourage you!