The passage you are referring to is the one that opens this incredible American classic, and refers to the advice that Nick received from his father:
"Whenever you feel like criticising any one," he told me, "just remember that all the people in this world haven't had the advantages that you've had."
Nick tells us the massive impact that this has had on him, as he is inclined to reserve making judgements, and he even goes as far to say that "Reserving judgements is a matter of infinite hope." We might want to question Nick on this, as he is certainly somebody that makes a number of judgements about people during the course of the novel, Gatsby included, but during this opening section of the story, our focus is placed on the way in which Nick's father "snobbishly suggested" and Nick himself "snobbishly repeats" that "a sense of the fundamental decencies is parcelled out unequally at birth." In spite of the word "snobbish," I do think that this is an accurate assessment of the matter, as life is a bit of a lottery, and depending on which family we are born in to, our lives will be shaped for the better or the worse. Sociologists call this life chances, and make much of the fact that those born into working class families, for example, have less opportunity than those born into higher-class families.