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.