If the film is not exactly where that cone of light reaches it's smallest point, then that point of the image is not in focus. Then that point is rendered on film as a disk of light, instead instead of as a point. In a lens with no spherical aberration this blur circle is an evenly illuminated disc. Out of focus points all look like perfect discs with sharp edges. There are no perfect lenses, so one usually does not see these perfect discs.
Real lenses have some degree of spherical aberration.
This means that in practice, even though all the light coming through the lens from a point on the subject may meet at a nice, tiny point on the film, that the light distribution within the cone itself may be uneven. Yes, we are getting abstract here, which is why some denser photographers refuse to try understand bokeh. Instead they tend to have more of the light collect in the middle of the disc or towards the edges. This is a greatly magnified blur circle showing very poor bokeh. Here is an example of a technically perfect evenly illuminated blur circle.