Hawthorn uses an especially expressive simile to describe the emotions that momentarily reveal themselves on the face of the stranger as he gazed upon hester

First of all, you need to know what a simile is. It refers to a figure of speech in which the author compares two or more things using words such as like or as. I managed to find two such similes in the part where the stranger sees Hester.

1. It was carelessly, at first, like a man chiefly accustomed to look inward, and to whom external matters are of little value and import, unless they bear relation to something within his mind. - this is the moment he saw Hester

2. A writhing horror twisted itself across his features, like a snake gliding swiftly over them, and making one little pause, with all its wreathed intervolutions in open sight. - this is the moment when he recognized her; personally, I'd choose the second simile because it is more memorable