Metastability in electronics is a description of a digital electronics system's ability to persist for an unbounded time in an unstable equilibrium, also called a metastable state. This means that the system is not settled in a stable state (i.e. logic level) of '0' or '1', but rather is in an quasi-stable intermediate range. Metastability can occur in a few circumstances:
when two asynchronous (i.e. not simultaneous) signals combine such that their resulting output is not stable.
In electronics, metastability is often treated as a problem because it can cause unpredictable behavior and even system failure in digital devices, particularly when data signals are being transferred between circuitry in unrelated or asynchronous clock domains. This phenomenon is commonly known as a "glitch".
Ideally, all logic designs could be synchronous and all data inputs tied to the system clock. If this were the case, metastability would not be concern. However, data is often asynchronous with respect to the system clock in reality, so designers must consider the possibility of metastability.
Synchronizers are one tool that designers use to decrease the probability of a system entering and getting stuck in a metastable state by helping order the signals in asynchronous circuits.
Metastability - Advanced Logic Design and Switching Theory
Wrapping One's Brain Around Metastability