A backplate is usually a live action or scenic background which is filmed on location when it's not physically possible to bring the foreground action and the desired background together in the same place. You'll often see it used to place extinct or fantasy creatures in a landscape as in "Jurassic Park" or "Walking with dinosaurs". The 3D computer generated images (CGI) of the creatures are combined with the backplate in the computer to create the final composited scene.
The technique is also used to place actors or presenters in a scene by filming them against a green screen. The green background is then replaced with the backplate in a process called keying.
Also known as a Plate Shot