Teleprompter definition: 

Teleprompter is an useful aid in the studio, video recording, large lectures and some other occasions. The teleprompter's main function is to prompt the text, lines and other content, so as to ensure that in the recording of video or speech, there will be no embarrassing situation such as forgetting words and dropping sentences.

Not only that, but what the speaker likes most is that the speaker can face the camera while reading the speech.

The principle underlying is a one-way mirror!!