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Metastability happen at a flip-flop when the data is switching while the clock is also switching. Depend on the design of flip-flop, there is a finite probability that the output of the flip-flop will stay somewhere in between power and ground for a long period (long is relative to the frequency of the clock. A few tenth of millisecond is very long) before it will goes high or low.
There are flip-flop that is design to reduce the probability of this happen.
The standard solution is to limit the fanout of the flip-flops that could goes into metastable to 1 (usually this flip-flop is receiving asynchronous data, like pressing of a keyboard button, etc), and is connected to another flip-flop in a way called synchroniser (please check google for synchroniser). "Metastable" means that different input pin connect to the same net intrepret the logic signal differently. This might crash a state machine as this is not considered during design.
Metastability occurs is what when there is a setup or hold time violation. So in this state the output of a particular flipflop is in metastability(either 0 or 1) The FF doesnt give an appropriate output. It stays there for few time and then returns to a normal state. Here its metastable recovery.
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