Sadegh.j,
Metastability is a condition in a logic device with memory, for example, a flip-flop. The condition arises when the input setup and/or hold times are not met. When the clock is applied, the delay from clock to output is undefined. It may take ordeers of magnitute times the specified dely for the device to switch. Or, the output might even oscillate betwen "0" and "1".
Regards,
Kral