Stanley is a newcomer to the camp, while Zero has already been there for some time. It seems that Zero has a couple unusual qualities. The first is that he rarely speaks and interacts hardly at all with the other boys. As a result of this, he is ostracized and bullied. Even his name, Zero, comes from that—he is worth nothing, of zero interest to his fellow campers. His other unusual quality is that Zero is remarkably good at digging holes.
When Stanley arrives, he starts at the bottom of the totem pole, along with Zero. Therefore, they already have something in common. Stanley sympathizes with Zero. When he discovers that Zero cannot read, Stanley befriends him and happily teaches him how. In so doing, he finds that Zero knows and understands much more than he shows to others. In return, Zero helps Stanley dig holes—an activity which is one of Stanley's shortcomings.
The most significant challenge to their friendship is the other boy campers. Stanley gains some status among them because he refuses to snitch, but the other boys cannot forgive his friendship with Zero. This is particularly true after Zero starts helping Stanley with his holes. As a result of their friendship and help for one another, Stanley and Zero both suffer. They are bullied and targeted by the boys, who are encouraged by a camp counselor. It is a test and a testament to the strength of their friendship.