From Bulbapedia, the community-driven Pokémon encyclopedia.
Lagoon Town (Japanese: ラーグンタウン Lagoon Town) is an anime-exclusive location in the Kalos region. It is located between Shalour City and Coumarine City. It has a theater where Pokémon Showcases are held.
Ash and his friends arranged a meeting there with Shauna, who wanted to tell them something. When they met up in Dreaming a Performer's Dream!, Shauna informed them that Kalos Queen Aria would give an exhibition performance at the Pokémon Showcase Rookie Class held that day.
After Aria's performance, a wild Pancham ran onto the stage and interrupted the performance of Kayleigh. Serena would later catch the Pancham.