chapter 1 opens with them standing before the prison. what are they waiting for?

They are not waiting for anything or at least we never find out. The point of this dreary scene is to establish that even in a Puritan stronghold; the virgin soil was cleared for a prison and a graveyard. Death and sin are the two things the Puritans could not escape.

The crowd is waiting for Hester Prynn to take her place on the scaffold--a part of her punishment for adultery. It is the first time she has been seen since she was first put in jail. She has since given birth to her daughter, and she has fulfilled her obligation to sew and wear a scarlet letter "A" that stands for adultery.