A body double is using another person to stand in for/play the part of a person who is unable to do so. Usually they are used in acting for dangerous stunts where they will have another person of the same build and height and relatively similar features and hair to stand in for an actor who is unable to do the stunt.
In your case I’m assuming you’re reading up on the recent ‘Khashoggi’ case where the Turkish officials report that a body double was used for Khashoggi (meaning another person of the same build, height and looks, pretended to be Khashoggi) so that it would appear that Khashoggi was alive and left the Saudi consulate through the back door - which we now know he didn’t!