This novel's setting is what is referred to as "The Jazz Age" - a period of time in between the two world wars (after WWI and before WWII). During this time, American society became very focused on materialism. It was the industrial age and many new things were being invented, making life much easier and resulting in a lot more free time for people than during the prior agricultural age. More people lived in cities, and there were phonograph records, automobiles, etc. -- modern conveniences were beginning to explode. Rich people had a great deal of wealth, and spent money in an uncontrolled way. People purchased things on credit and pretty soon, all of this extravagance led to the great Stock Market Crash in 1929.
As you read through the novel, notice how Tom, Daisy and Gatsby live compared to how Nick lives. Notice the differences between East Egg and West Egg. Nick is from the Midwest, remember, and Tom and Daisy are from the East Coast. Gatsby is also from the Midwest, but he WANTS to be associated with the rich East-Coasters, hence the tragedy of his life. Nick is constantly evaluating how shallow Tom and Daisy's lives are (and their friends, parties, etc.). Nick feels pity for Gatsby when he tries to live up to all of this and cannot, because he really is not part of Tom and Daisy's world. At one point, Nick will say that Gatsby was better than the whole lot of them.
Towards the end of the novel, Nick will say that Tom and Daisy were "careless people." I don't want to ruin the novel for you by giving you too many details, but if you keep these things in mind as your read it, you will undertand the criticism of the affluent American society during the Jazz Age.