Answer:
Nick describes the "valley of ashes" that is the area between the rich suburb of West Egg and Manhattan. This is the gray and dirty part of the borough of Queens that you drive through to get from Long Island to NYC.
Above this bleak, smoky, unpleasant landscape is a giant billboard advertising Dr. T. J. Eckleburg, an eye doctor. The billboard is a set of giant eyes that seems to be surveying or judging everything below.
Tom's mistress lives in this "ash heaps" area.
One day, when Nick takes the train with Tom to Manhattan, Tom suddenly makes him get off at a random stop to meet her.
They go to a garage owned by George Wilson, who seems to be in the middle of buying a car from Tom. Myrtle Wilson, George's wife, comes down to the garage. She isn't beautiful, but is attractive because she is plump and lively. Tom quickly makes a plan to meet her in the city. He and Nick leave, and Tom explains that George has no idea that Myrtle is having an affair with Tom.
Tom insists Myrtle meet him in Manhattan, so she boards the same train as Tom and Nick, but she sits in a different car to be discreet, and they then meet up at the station.
Step-by-step explanation: