In the zone defense each players has the main responsibility of a certain area.
If an attacker leaves the area, another defender takes over the defense in the new area.
The areas overlap each other and the defenders will therefore cover 1 1/2 player.
That means that if for instance the opponent's RB attacks the area between LW and LB, the defending LB must have the primary job to defend (1 man) and LW must be ready to backup (1/2 man), but be aware of her own attacker (the wing) at the same time.