The zener region describes the area on the performance curve (a graph of voltage across versus current through the junction) of a zener diode. The diode acts like a "regular" diode in the forward biased direction. When some 0.7 volts or so is reached, forward current begins to climb rapidly as voltage is increased (for silicon diodes.) But in the reverse direction recall that as the diode is reverse biased, a small amount of current will flow (because of minority carriers). This "trickle" of current will continue until the "zener voltage" is reached, and then the diode will begin to conduct heavily. On the graph, this is the zener region. Zener diodes can be made to breakdown at a specific voltage, and their ability to conduct reverse current can be increased by manufacturing a larger diode. That means there are a range of voltages and wattages of zener diodes available. Wikipedia has more information and that graph. Use the link provided to get there.