What does sexton tells Dimmesdale the large letter "A" in the sky represents?
The sexton tells Dimmesdale that the letter "A" represented the angel that Governor Winthrop had become after his death the previous night. He says,
"But did your reverence hear of the portent that was seen last night? a great red letter in the sky,--the letter A,--which we interpret to stand for Angel. For, as our good Governor Winthrop was made an angel this past night, it was doubtless held fit that there should be some notice thereof!"
Dimmesdale, of course, interpreted the letter to be a reminder of his sin with Hester.