Voix céleste is a fancy French term that refers to a type of organ pipe that is designed to imitate the sound of a human voice. Imagine singing "la la la" in a high and soft voice - that's how a voix céleste sounds on an organ.
The organist makes this sound by using a special pipe with tiny holes in it that let air escape, called a "spotted" pipe. When air flows through the holes, it creates a gentle, ethereal sound. The voix céleste is often used in church music to add a heavenly or angelic quality to the music.
So, a voix céleste is basically a special pipe on an organ that sounds like a soft, high-pitched human voice. It makes music sound more like it's coming from Heaven!