The question is talking
about the C of G being towards the rear of the envelope compared to near the
front - as opposed to out of the envelope on either side. Within the envelope,
the further forward (nose heavy) the greater the stability (good thing) but the
price is paid in greater wing loading induced by tail-down elevator to balance
the forward C of G.
This leads to higher
stalling speed (bad thing).
If tail heavy, by
contrast, stability is reduced (bad thing) but stall speed is lower (good
thing) as there is less need for the wing-load-inducing tail-down elevator.