In this scene Katie finds out she has a rare skin condition in which is commonly referred to as XP. This means if she is exposed to sunlight she will lilely contract skin cancer and die.
Katie is now much older and has been home schooled her whole life. As her graduation gift she was given her mothers guitar to help her pursue her dreams of being a musician.
Katie goes out that night to try out her new guitar at the train station near her house. Her crush Charlie shows up and listens to her play unbeknownst to her. When Katie notices him she ups and leaves so fast that she forgets her song book and Charlie keeps it.
Katie's best friend Morgan convinces her and Charlie to meet up. This allows for Katie to get her song book back and gives Charlie a chance to ask her out.
After Katie says yes her and Charlie hang out every night They stay out late at night so Katie stays safe but Katie doesn't tell Charlie about her condition.
One night as they stay out they go swimming in this lake. Katie's watch dies and she doesn't pay attention to her phone and she wakes up as the sun is rising. In a panic she runs back to her house.