With NAND a “0” or “1” is determined by the threshold voltage Vt of the cell.  The threshold voltage can be manipulated by the amount of charge put on the floating gate of the Flash cell.  Placing a charge on the floating gate will increase the threshold voltage of the cell.  When the threshold voltage is high enough, around 4.0 V, the cell will be read as programmed or a 0.  No charge in the floating gate, or threshold voltage < 4.0 V, will cause the cell to be sensed as erased or a 1.  So as charge trapping occurs it becomes more difficult to erase the cell.