To simplify much more than what is stated in the Poser manual, here it is.  The Focal Distance determines at what range from the lens you will have the sharpest focus.  For completely accurate calculations you would need to take into account the focal perspective of the camera, as well as it's maximum distance (yon).  I suggest not worrying too much about doing the heavy math.  The F-Stop, when altered with Focal Distance will increase or decrease the range of focal clarity (that is how much further away from the point of focus things will remain in focus).  Larger F-Stop numbers give a wider range of focus, while smaller ones give a narrow range.  Your focal field increases with F-Stop toward infinity the higher it goes in relation to the Focal Distance of the lens.  The further your Focal Distance and higher your F-Stop combined the greater your focal field will become.  The easiest way to get a feel for it is to run some render tests at different settings with simple objects spread throughout the scene.  Otherwise you will need to pick up a good photography reference book.

Both of these settings only have an effect if you have the render settings with depth of field enabled.  This is because it is a virtual camera, and without actually rendering depth of field, there is no focal plane, F-Stop becomes irrelevant, and there is no focal distance as everything is in focus.

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