What is the red edge of the picture taken by Sony 580 camera?

You accidentally pressed the focus assist button. Press this button, the red edge will appear on the displayed screen as you said, but it will not be recorded in the video. You find the auxiliary focus button and press it again, and the red edge disappears. English names of different brands may be different. Try to press the buttons one by one, and be sure to be near the manual focus. Because I have never used the camera you mentioned, the English names of surgeons may be different, but the meaning is the same.