A teleprompter, also known as an autocue, is a display device that prompts the person speaking with an electronic visual text of a speech or script. Using a teleprompter is similar to using cue cards.
The screen is in front of, and usually below, the lens of a professional video camera, and the words on the screen are reflected to the eyes of the presenter using a sheet of clear glass or a specially prepared beam splitter.
Light from the performer passes through the front side of the glass into the lens, while a shroud surrounding the lens and the back side of the glass prevents unwanted light from entering the lens.
Mechanically this works in a very similar way to the "Pepper's Ghost" illusion from classic theatre – an image viewable from one angle but not another – and the concept may have similar origins.