What does it mean if your cat has a worn patch of fur on her leg?

I would guess there is something bothering your cat in this location - it could be anything from a flea infestation to a skin infection to an underlying nerve problem that's causing a weird feeling there. Without more information and/or seeing your cat, it's hard to narrow down the cause - a veterinarian visit may be a good idea, particularly if the area is red, swollen or oozing anything unpleasant.