Shepard tones are an aural experience produced by playing several note together that rise or fall together. The top note fades out, and a new bottom note starts rising (or vice versa), creating an auditory optical illusion that seems to continuously get higher and higher (or lower and lower) eternally.
You can also play both an ascending one and a descending one at the same time. This particular version has sliders for each component sound that you can adjust louder or softer to explore how the noise works.
Take note: This is not a pleasant sound. Some people also experience nausea or anxiety when listening to it.