Decades ago, my dad posted this question to me: "What is Music", when I was auditioning for Australia's Newington College full scholarship. In case they asked, but they didn't.
Nevertheless, it is an important question. Because people would give various answers. Some good, mostly bad ones. I would say.
It's a philosophical question. Great minds have tried answering it.
For now, I'd settle with Music is the art of time. Some would agree, I'm sure.
TwoSetViolin gave some basics on What Makes a Good Classical Music Performance:
Hillsdale College's Hyperion Knight's free course on Classical Music History is another introduction into music.