The exposition is the name given to the initial part of the plot structure, in which the main characters are introduced and their conflicts. It is introductory in that it sets up the rest of the rising action that will follow as the conflict works itself out.
In this great novel, we are introduced to Jess, who is obviously brought up in a family with lots of issues, not the least of them being the poverty they face. As he gets ready to school, he thinks about the race he is hoping he will win. However, the race is won by a new girl who is obviously not from this area, and Jesse has to cope with his feelings as he does not win the race. The resulting friendship between Leslie and Jess and how they survive together in school and their imagination occupies the rest of the novel.