Zero (Hector Zeroni) told the boys' supervisor Mr. Pendanski that he liked to dig holes. This is ironic because digging holes all day long in a parched, desertic land in the middle of nowhere (ironically called 'Camp Green Lake') is more of a punishment than anything else (although Mr Pendanski tires to convince the boys that it "builds character").
Zero and Stanley Yelnats become best friends at this "reform camp" and strike a deal that if Stanley teaches Zero to read, Zero will help Stanley dig his daily hole. This they do, but that Hector learning how to read is not all that happens as a result. To find out the big surprise, read this delightful book. It is not only great fun but raises a lot of important issues as well. (There is a film out, too, if you want to take a shortcut.)