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Roger,
Prime focus - this is the natural focus point for the telescope optics - in the case of a reflector the focus point without using additional optics (Barlow lens etc.)
The issue mentioned refers to the clearance distance between the focuser (at minimum extension) to the focus point - sometimes called the "back focus"
If this distance is too short then you are unable to fit say a DSLR camera body in the "gap" and get the focus point onto the CCD chip.
It can also have some impact when some eyepieces are used...again insufficient (generally inwards - towards the tube) space to bring the eyepiece to focus.
Hope this helps.
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