Applejack has been learning to play the tuba ever since she was being switched from the euphonium when she was Applebloom's age. Now that she's been playing it for several years, meaning that she's an advanced tuba player, Applejack has been asked to play in an all-girl brass band, called the Brass Beauties, with Pinkie Pie, an advanced trombone player, Rainbow Dash, an advanced trumpet player, and Scootaloo, an intermediate cornet player. She has accepted the request and starts playing with them at almost every street they're assigned to perform. As close friends, they enjoy being with each other while playing.
Now, Applejack is being asked to perform on stage at the Canterlot Auditorium, playing her tuba solo. But she isn't completely alone, because Octavia Melody, Watermelody, and Derpy Hooves are being asked to play their instruments on stage behind her, accompanying Applejack's tuba solo, and Sweeten Sour to conduct it. This is really formal and special for them, especially for their friends, and the Apple family.