The Scarlet Letter is a novel concerned with the inner workings of the soul as it deals with sin and shame. Hester was an adulteress whose sin was made public. The man she loved, Arthur Dimmesdale, hid his sin and continued his role as revered minister to his church flock. The day on which this conversation took place was Election Day, one of the grand ceremonial days in the town. Dimmesdale was chosen to deliver the Election Day sermon, and the former lovers had plans to leave for England immediately following the event. As he walked through the crowd on his way to the church, Arthur was a changed man. The formerly weak and frail man who was continually clutching at his heart was walking erect and strong, with an apparently renewed energy and vigor. He passes by Hester and Pearl without a word or a glance, though Hester longed for some signal of recognition. Mistress Hibbins joined Hester and Pearl, creating a kind of aura which made others keep their distance as they held the private exchange you're asking about.
Mistress Hibbins, perhaps with her witch-like sense or perhaps something more, has figured out that Hester and Arthur had a meeting in the forest and because of it things are changed. She said to Hester:
"Many a church-member saw I, walking behind the music, that has danced in the same measure with me, when Somebody was fiddler.... But this minister! Couldst thou surely tell, Hester, whether he was the same man that encountered thee on the forest-path?"
She is clearly indicating that she knows Arthur, as a godly man, has sold his soul to the devil in order to escape his guilty conscience. Hester, of course, tried to deny and deflect; however, Mistress Hibbins would not be deterred:
"Fie, woman fie!" cried the old lady, shaking her finger at Hester. "Dost thou think I have been to the forest so many times, and have yet no skill to judge who else has been there?
She clearly knows that Arthur has been lost to the devil. Pearl, of course, wants to know all the details, but none are forthcoming. The unwanted visitor walks off, but Hester is shaken. This encounter clearly foreshadows trouble to come.