The camera lens, in many respects resembles the Human eye. Now the pupil of the human eye is that black circle in the center that expands and contracts involuntarily in response to the ambient light level. The larger the entry way diameter, the larger the surface area, the more light the eye can gather. The average human eye expands to about 7mm diameter under feeble light conditions.
All optical systems are plagued with defects called aberrations. To mitigate this the lens maker is forced to use multiple lenses constructed using different densities of glass each with a different shape (figure). Thus when we look into the lens from the front we get a view of aperture blades. This circular orifice is called the entrance pupil. Contrariwise, when peering into the lens from the rear we are looking at the exit pupil.
In both views, we are looking at the circular opening at the aperture stop, however they likely appear different in size because we are looking at this iris through lenses that likely magnify. The significance: It is the center of the front entrance pupil that defines the viewpoint of the lens. From the center of the rear pupil we trace the path of the un-deviated light rays that project an image on film or digital sensor.
Additionally, the diameter and distance from the exit pupil to film or digital sensor determines the size of the circle of good definition of the lens. In other words, will the amount of vignetting be tolerable?