This quote comes from Nick's opening introduction to the book where he tells the reader about a bit of advice his father gave him in his younger days that has stuck with him ever since:
"Whenever you feel like criticizing any one," he told me, "just remember that all the people in this world haven't had the advantages that you've had."
As a result, Nick tells the reader, he has been inclined to "reserve all judgements," and this policy with regards to meeting people has meant that he has been exposed to many "curious natures" as well as meeting a "few veteran bores." However, Nick maintains, that in spite of those "veteran bores" reserving judgements is overall a good thing, as it is a "matter of infinite hope." Nick's father's advice is all about the dangers of jumping to conclusions about people, and thus Nick begins his account of the life of Jay Gatsby by saying that this is something he tries not to do and therefore encourages the reader not to do otherwise. Reserving judgements is such a good thing because as long as people reserve judgements it means that the true wonder of characters such as Jay Gatsby, who might be written off very quickly otherwise, can be discovered and dwelled upon.
However, at the same time it is important to realise that although Nick says he lives his life by this creed, there is evidence to argue that he is actually very swift to make judgements of characters, as his presentation of the people who populate Gatsby's parties demonstrates. Reserving judgements may seem to be a symbol of "infinite hope," but it is one that the narrator himself does not entirely live by, and Nick's narration and his own partiality must therefore be closely scrutinised, particularly with regard to Nick's feelings about Jay Gatsby himself.