Lets break it down.... You are still wondering, What is a Photo Double?
A photo double must resemble an actor as closely as possible in height, build, hair color, and complexion. They often need to look like they could be the actor's twin. A photo double can be used when a principal actor is in a scene but is not the focus and is essentially functioning as a Background Actor.
So the long and short of it is, the photo double must resemble the principal actor enough that they can double as him in scenes and you think you are looking at the principal actor but in reality it is the photo double. This is used alot with child actors due to legalities and rules that protect them. They can not work on set as long of days as an adult actor so by using the photo double it lengthens the amount of scenes they can shoot that day of that character.
Now there is a similar role of a person who is a "Stand In" that is often confused with photo double. They are not the same thing.
Stand-Ins (often called the second team) take the place of a principal actor for rehearsals, camera blocking, and lighting setups. They do all their work behind the scenes and are never seen on camera. Stand-Ins get to work closely with the actors and production crew to help ensure everyone is on the same page for every shot.
A stand in gets used almost every scene because they fly in and are in position before the first team of principal actors come in.
Most times a photo double has to go to wardrobe for a fitting before they start filming. They have to wear the exact outfit that the principal actor wears for each scene.