To agree with Nick's father is to agree also that people are shaped by their past. A person's virtues and vices stem from their particular upbringing, their circumstances, and their challenges.
This premise is easy to agree with because we can all see how we have been shaped by our experiences and simply how life has taught us lessons.
I agree with what Nick's father says because I believe that some kids grow up in better situations than others. I am not just talking about wealth and things like that. I'm talking about their home setting, their inborn traits, and things like that. For example, I tell my daughter that she should not think that she's better than people because she gets good grades. She has to realize that she was born smart and was then raised in a family where her parents were able to help her learn lots of stuff. It's partly to her credit, but much of it is due to things that she had no control over. So I totally agree that it is wrong to judge others because we do not know the totality of their life experiences.
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.