What does Sal understand about that phoebe and prudence don't?

Sal understands the importance of forgiveness and moving on from the past, which Phoebe and Prudence struggle with. Sal learns to let go of anger and resentment, while Phoebe and Prudence hold onto grudges and are trapped by their emotions. This allows Sal to find peace and closure, while Phoebe and Prudence remain stuck in their pain.