In a typical exposition, the author might give background information, events leading up to the beginning of the story, and/or descriptions of the setting and characters. In "A & P," Updike's narrator, Sammy, uses the exposition to give very detailed descriptions of the three girls entering the store. He briefly mentions an older patron as well. And although this is brief, it is important to understanding Sammy's mindset at this stage of his life.
While he is captivated by the three girls, he makes a mistake and rings up a box of crackers twice. According to Sammy, the older woman gives him "hell" about it. He then notes that had she been born in Salem during the witch trials, she would have been burned. The old woman may in fact have overreacted to Sammy's mistake. But she also simply did not want to pay for something twice. Sammy's description of her as a witch, paralleled by his idealization of the girls shows a clear dichotomy. He is antagonistic toward older generations and idealizes youth. This perspective will play into his decision to stand up for the girls by quitting his job.
Sammy's description of Queenie is quite detailed. It is mostly superficial and therefore an objectification. In the exposition, he briefly wonders about her personality and her thoughts:
You never know for sure how girls' minds work (do you really think it's a mind in there or just a little buzz like a bee in a glass jar?) but you got the idea she had talked the other two into coming here with her, and now she was showing them how to do it, walk slow and hold yourself straight.
Sammy is mostly interested in how Queenie looks. Although his comparison of her mind with a bee buzzing could be considered a playful mockery, it is pretty misogynistic. Sammy is mainly taken by outward appearances. This shallow part of his thinking makes the act of quitting more complex. He does stand up for the girls out of principle. He is siding with the younger generation. He is opposing the older generation. He believes that with his quitting, there is something admirable about it but it is also going to make things difficult. Given that his infatuation with Queenie is largely superficial, his admirable action lacks real substance. He is not standing up against some huge injustice. His choice is more of an existential motivation. In other words, his rebellion, if we can call it that, is based on his own subjective concerns. These happen to be concerns of youth, desire, and pride.