As The Great Gatsby opens, Nick Carraway explains that he is kind of person who people want to confide in. He says he often has tried to sidestep these revelations from people by, for example, pretending to be asleep or cracking a joke. Nick then says that:
Reserving judgments is a matter of infinite hope.
He implies that he avoids hearing people’s secrets because he wants to retain his hope (or faith) in them. He doesn’t want to hear what will cause him to judge the other person harshly. As long as he can avoid doing that, he can think better of the person.
In a general sense, to “reserve” a judgment is to withhold it. It means not to make it, not to decide. It also carries the connotation of waiting, of letting as much time elapse as possible before making a decision or judgment about another person. In this sense, it is similar to making a dinner reservation: you will show up to eat at a restaurant, but after some time has elapsed. Nick is saying he will come to a conclusion about someone, but not until later. In the meantime, he can hope for the best from that person.
Nick’s habit of reserving judgment is opposite to the behavior of people who jump to conclusions or judgments about others on very little evidence.