All photographs taken by the same camera should be imported with the same rotation (either all portrait or all landscape). The standard is to define cameras with landscape orientation (i.e., the format width is larger than the format height) but if you take some photographs with the camera rotated clockwise or counter-clockwise 90 degrees you will have a portrait photograph. This photograph must still match its camera and be imported in landscape rotation. This means you would have to view the photograph with the scene lying on its side (i.e., not upright).
The Photo Rotation setting fixes this. You import all your photographs in landscape (more pixels wide than they are high), and use the rotation on the photo to give you a view that is upright and easier to work with.
When the Rotation property is selected a down arrow button is shown which provides access to the available rotation options.
Each photograph has its own rotation setting and this setting is saved with the pmr so each time you open this photo it will open rotated the same way.