Perspective refers to the conditions in a still image or video caused by the placement of the viewpoint (often referred to as the "camera",even if there is no actual camera, such as in a drawing). Obviously, objects closer to the camera appear larger than objects farther away. This can often be used to trick the human eye, creating optical illusions.
In the OBD, perspective is mostly important when doing scaling calcs. The typical method of scaling works by taking a known size value (say, the height of a character, often given in databooks or other supplementary materials), and measuring and converting it to pixels in an image, then using a ratio to compare it to the size of the thing in the same object you are trying to scale. Due to the errors introduced by perspective, this will not work if either of the objects being compared is too far in the foreground or the background of the image. Effective scaling of this type can only be done when both of the objects being compared are roughly the same distance from the camera.
Certain posters continually fail to grasp this concept.