As the mother of a five-year-old who’s never gotten over her “why?” stage, I answer a lot of questions. I’ll start to occasionally post the more interesting ones here. I’ll always cite sources and make no guarantees that the answer is complete, or even, you know, right (though I will do my best). This one actually comes from my brother (again).
According to IMDB, it’s the “Canzonetta sull’aria” from Mozart’s Marriage of Figaro. An aria is a song within the opera that showcases the singer’s talent. The overture is the sort of summary of all the major themes of the opera that’s played at the beginning before the action starts.