The correct answer is - overture.
The overture, also commonly known as intro, is the piece of music that is played as an introduction to the main piece. It is the one that starts the show. This piece of music can range a lot in length, though most commonly it is short, around one to one and a half minutes.
It is a specifically made piece of music that has a purpose of introducing the listeners to what they can expect, but it is a short piece that manages to incorporate the vibe, the feelings, the theme, of what will come.