In the great gatsby: what is the symbolic significance of nick mentioning that there was an “autumn flavor in the air” (153)?

In the particular scene you are referring to, happens in Chapter 8, which is the second-to-last chapter of the novel.  Even though the setting within which this phrase occurs happens in the morning, which one usually relates to new beginnings, the word “autumn” serves to symbolize the ending of things the way things sort of die off in autumn and sort of pause—leaves falling off trees and animals beginning to hibernate.  Thus, because Nick’s tale is coming to an end, it is symbolically significant that the mental image of the fall season is presented to readers as a sort of foreshadowing. 

? ? what can you explain this to me i am so confused

a dellusion is athing where your mind sees stuff