False vacuum refers to an unstable state of vacuum. Just as a radioactive nucleus may radomly decay via tunneling, as in beta decay, a much larger portion of the nuclear energy may be suddenly released in a nuclear explosion...A false vacumm may also suddenly undergo a transition to a lower energy state:
From Dr Kaku, HYPERSPACE, PAGES 209-211
A false vacuum is a theoretical concept in which a state of maximum symmetry exists, like right near the big bang when all forces were one, but it's at an unstable (false) higher level of energy...so it's a high energy vacuum state likely to randomly break down to a lower energy state. This breakdown is called a phase transition and corresponds to symmetry breaking... dissolution of a single unified force into many...
(Think of a pencil balanced on it's point on a table...unstable high potential energy, high symmetry...it falls: lower energy, symmetry broken, stable until it rolls off the table.)
"Regarding superstring theory, physicsts assume (but cannot yet prove) that the original ten dimensional universe was unstable and tunneled its way to a four and six dimensional universe"
False vacuum transition is probably the same thing as the cosmological constant...maybe quantum tunneling can change the state of a nascent quantum environment...as just after the big bang...and power the expansion of the universe...
The false vacuum may be the previous generation of vacuum from which the current generation of Universe arises. You can also call it the vacuum of the pre-inflational era.
The 'false vacuum' is a reference to the notion that the cosmological constant is a function of various fields and that our universe's state, though stable over long periods of time, isn't totally stable and that there exists another semi-stable field configuration with a lower cosmological constant...maybe we'll transition to another form of dimensionality and time in a another sudden change...and we'll all disappear in the creation of the next stage universe.