Symbolism, Imagery, Allegory
In this case we're referring to the Thanksgiving drive that Neil and Grace take, which could be an allegory for lots of different things: loss of innocence/becoming an adult, disappointment, redemption…the list probably goes on, and it's definitely worth thinking about why those ideas might be represented by the drive.
But, we're going to concentrate on this passage:
What she [Grace] remembers is…hardly distinguishable from her idea, her fantasies at that time, of what sex should be like. The fortuitous meeting, the muted but powerful signals, the nearly silent flight…an airy surrender, flesh nothing now but a stream of desire. (174)
Go ahead and marinate on that for a moment.