Serena receives the blue ribbon she will wear as part as her new outfit starting in the following episode.
This marks one of the very few times Ash has ever given a gift to one of his female friends (he gave a Beedrill to Casey in Bug Catching Tournament! Capture a Pokémon at the Natural Park!! (2000) and a flower to Iris in Satoshi and Iris's Severed Relationship!? The Path of Separation!! (2013)). It can be noted that unlike when he gave those gifts to Casey and Iris, he gave Serena the ribbon without any advice from another person.
Just like he did in Satoshi and Iris's Severed Relationship!? The Path of Separation!! (2013), Ash wants to catch a Dunsparce, but is unable to, since he has no Poké Balls on hand.
Dub title: Under the Pledging Tree.