If you are referring to the philosophical thought that has been pondered upon for centuries, then it actually refers to the question - Can something exist without actually being perceived? If you fall or make a sound in a forest where nobody can hear you or see you, did it actually happen? There is no definitive answer to this, as it is open for discussion.
However, if you are referring to lyrics, then it is a song from Dear Evan Hansen, called Waving through a Window.
