What is a false vacuum?

False vacuum and true vacuum form a metastable system. Metastable state is a state which does not change with time but is susceptible to falling into lower-energy states with only slight interaction.

It is analogous to being at the bottom of a small valley when there is a deeper valley close by. True vacuum is the name applied to a condition of the true lowest energy state of the vacuum. False vacuum refers to a condition with an elevated vacuum energy density. The two conditions are separated by some energy barrier – which is why they form a metastable system. If the current universe is in a state of false vacuum, it is conceivable that some experiment would concentrate enough energy into a small enough place to coax the vacuum over the energy barrier and into the true vacuum state. This could perhaps be catastrophic.