In the third paragraph of the novel, Nick Carraway states that he's "inclined to reserve all judgments." In other words, he is saying that when people talk to him, he tends to listen quietly and not say what he thinks. He explains that because of this personality trait, people often open up to him and tell him secrets that they would not normally tell other people.
Nick is the classic observer: he is the type of person who stands on the edge of a scene, watching what other people are doing. He is a good listener rather than a talker.
Nick does judge, calling people "bores" and saying they often lie by omitting parts of their stories (what he calls "obvious suppressions") and saying that he has to remind himself of his father's words, that everybody isn't born with the same advantages.
Although Nick does judge, he keeps quiet about his feelings and doesn't openly criticize others. Therefore, because he seems safe, people tell him things.
Nick is explaining how he knows so much of the Gatsby-Buchanan saga, even though he was primarily an outsider looking in.