The opening chapters of the novel serve to introduce the major characters of the story and the major themes of the story.
Nick first introduces himself and, via commentary, introduces Gatsby and associates himself with the wealthy man. Nick is repelled and won over by Gatsby and shown in the opening pages of the novel to be a symbolic and larger-than-life figure.
[Gatsby had] an extraordinary gift for hope, a romantic readiness such as I have never found in any other person and which it is not likely I shall ever find again.
Nick's world view is depicted here in its unromantic maturity. This quality of perspective in Nick is, in part, what Nick suggests the story will be about.
Tom and Daisy are introduced early in the novel as well; Tom in his brutality and hubris; Daisy in her splendor and basic insincerity. Jordan is introduced as well and Nick's eventual relationship with her is strongly suggested in the opening sections of the novel.
Gatsby's wealth is described in these pages as are Nick's short-lived but telling ambitions (to become a "well-rounded man" and an intellectual). Nick's intellectual vanity is ironic and serves to undermine his claims to honesty, humility, and moral integrity. Nick is a moral person, essentially, but his early admission of ambition aligns him with the same people he later condemns. Thus a complexity of character and perspective are established in Nick Carraway.
In terms of plot, this section covers Nick's arrival on the east coast, his visit to Daisy and Tom's house, and an introduction to Tom's mistress Myrtle.