This is one of those awesome, happy, everything-comes-together endings. It makes you want to hug your parents and text your friends to tell them they're the best parts of your life. Not kidding.
In the last paragraph, while Otis plays guitar and Opal's new friends gather to sing, Opal sucks on a sweet and sad Littmus Lozenge which feels "like a flower blooming" in her mouth, and she listens carefully to memorize the words of the song being sung (26.33). It's basically a Norman Rockwell.
So what does it mean?
Otis was shy, backward and lonely. Now he's opened up enough to share his musical gift with others. Also, each new friend took a little of Opal's emptiness away. Together, they help each other heal and find strength.
By the end of the novel, Opal has learned how to survive her sorrows, and the key to her survival is not a package of medicine floating down from the arena sky. It's finding sweetness in love and friendship with others.
She's also learned about the hidden sorrows that people face. This helps her love them despite their mistakes. In the last scene, everyone finds sweetness together as rain falls outside.
Here's a little shout out to when the preacher told Opal ten things about her mama. After he did, she went to her room and memorized them so she'd never forget them. She wanted to burn 'em into her brain so she could "hold on to her tight and not let her get away […] again" (4.22).
While Opal realizes it's okay to let go, she seeks to hold on to this moment of joy, love, and friendship so it will never get away from her again.