Vacuum decay, or vacuum metastability event is the possibility in simple terms that the universe itself is not in in its ground state. If that’s true, it might spontaneously change to its real ground state. Doing so will change fundamental things like the strength of electromagnetism, the weight of particles and so on. It would literally destroy everything in the universe, and we couldn’t exist in what’s coming after.
Good news, we’re confident, that’s probably not going to happen.