Simplified,a vacuum is a lowest energy configuration around which you can expand your theory. A true vacuum is a global minimum of the energy and a false vacuum is only a minimum locally (think of an egg in an Eagle's nest on the side of a mountain - it is a local minimum of potential energy for the eggs, but they would have much less potential energy if they fell down to the valley).
Metastability means a state is almost stable and may be treated as such in several respects. However, even for a local minimum of the potential, there is a probability of tunnling through the energy barrier into the true vacuum region.