False vacuum and true vacuum form a metastable system. True vacuum is the name applied to a condition of the true lowest energy state of the vacuum. False vacuum, on the other hand, refers to a condition with an elevated vacuum energy density. The two conditions are separated, of course, 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!
The idea hinges on two assumptions: first, that false vacuum really exists. It is supposed by theory, but that doesn't mean it really exists. Second, we assume that our experiments are significant. In reality, astronomical events like black hole mergers and so on occur that make our little particle physics experiments look like a kid's sparkler. There's really no worry.
- Warren