Kristi as a symbol for normal society
Catherine sees her neighbor as an opportunity to encounter regular people with regular lives. That's not exactly fair to Kristi, and it's not exactly a good worldview, but it's perfectly normal for a kid like Catherine to do this. So for Catherine, Kristi represents normalcy and regular life, as opposed to Catherine's unique experience of having an autistic sibling.
Jason's silence as a symbol
Jason suffers from a disability that prevents him from being able to communicate, and therefore, he isn't able to speak on his own behalf, meaning that he might have to suffer indiscriminately. But fortunately, Catherine helps to provide him notecards that help him speak for himself in a way.
This represents the helplessness of people who are differently abled, and the it draws attention to the fact that with community support, disabilities aren't necessarily impossible situations. There is still value and meaning to Jason's experience, and letting him share that is one of the main meanings of the story.
OT as an allegory for the difficulty of life adjusted to the needs of others
By attending David's OT with her family, Catherine participates in his needs, instead of just her own, so OT is like an allegory for the systematic process of learning to help others and love people well.
The dance as a symbol for society and social success
By dancing with Jason, Catherine shows that she is not embarrassed to be associated with the disabled. She finds community and celebrates that community among her peers, finally identifies a true friendship that she can take pride in.
Jason's guitar as a symbol for love
A guitar is a beautiful instrument, and the gift of music is a sweet one, but Jason's guitar has extra meaning attached to it. It's Catherine's sacrifice to him, and it gives his life a new dimension. Instead of focusing on his disabilities, Jason has the option to focus on guitar, an outlet for him. In a way, it is a symbol for the kind of hope that love inspires.
