The setting of The Great Gatsby are a pair of neighborhoods divided by a bay, both in the shape of eggs. The two eggs are separated in status by the family history of the people inhabiting them: the East Egg is considered high society because the people living there are mostly "Old Money," from wealthy families; the West Egg is considered lower -- but still high -- society because the people living here are mostly "New Money," from recent earnings or windfalls. Nick, the narrator, has rented a house in the West Egg and is trying to fit in with the upper-class people and parties of the East Egg, even as he scorns the shallow mindsets of East Egg residents.
I lived at West Egg, the—well, the less fashionable of the two, though this is a most superficial tag to express the bizarre and not a little sinister contrast between them... My own house was an eyesore, but it was a small eyesore... Across the courtesy bay the white palaces of fashionable East Egg glittered along the water...
(Fitzgerald, The Great Gatsby, mrbye.com)
Nick's initial assesment of the relative merits in either Egg is an example of his desire to mix with the upper-class. However, his money came mostly from his own hard work, so he is of a lower social class than the East Egg residents. Crossing the "courtesy bay" and becoming friends with Jordan Baker raises his status, but remaining friends with Gatsby even after Gatsby's social status falls lowers it. Nick finds that the people he likes are on the West Egg, including Gatsby, while the people he does not like are on the East Egg.