Well, the way I understand it, it's kind of easy to think about. YOu remember those ray tracing diagrams you drew in physics, with lines coming from an object, bending through a lens and forming an image.... those were very simplified.
Just think of it that rays from any single point of an object will bounce off in all directions and pass through every area on the lens (like a ray will bounce off the top of an object, hit the TOP of the lens, and focus onto the image plane, just as a ray from the same point will bounce off and hit the BOTTOM of the lens, and RIGHT and CENTRE, and everywhere in between, all converging to a single point on the image plane. That is all points of an object will have light travelling though all points in the lens.
Now that's where apertures come in and why a WIDE aperture actually causes fuzziness (or out of focus blur) - the lines from a single point DON'T converge on the image plane, but rather in front of or behind it....thus at the image plane, it's just a circle (because the aperture is round), known as a circle of confusion. Wide apertures capture the light from other objects that don't converge. By stopping down or narrowing the aperture, you cut away the light that doesn't converge (i.e. the light that passes through the periphery edge of the lens), leaving only light that is more converged, thus, more 'in focus'.
Well, to my basic understanding anyway.
