Fading in means that the volume of the sound gradually changes from silent to normal when playing a song, and fading out means that the volume of the sound slowly changes from normal to silent. When switching songs, the volume of the music being played It doesn't stop abruptly, and the music that's about to play doesn't come out of nowhere! The sound of the previous song slowly decreases until it completely disappears at the end. When the next song starts, the sound goes from nothing to slowly rising. This is a more user-friendly setting!