The fundamental idea is the "false vacuum."
The idea is that the lowest energy state of quantum field theory is not the "everything zero" state. One common image is the "Mexican hat potential." Here, the lowest energy state forms a "ring" of non-zero values. The fields can have a lower energy state by moving away from the zero-field state. However, they are only lower energy when all of the fields in a region move the same small amount away from zero.
There are motivating examples of this. Consider a material in which the atoms each have a small magnetic moment. These atoms will be at lower energy if they all align, as compared to if they are all randomly oriented. This will mean that a non-zero magnetic field is lower energy than a zero magnetic field. However, if a large part of such a material "freezes in" with the atoms pointing in one direction, and a neighboring part does so with the atoms pointing in a different direction, there will be a boundary. At this boundary, the atoms will be unstable, sometimes flipping one way, sometimes the other.
If one region starts to flip-over to match the next region, then the energy released can be significant, and it can propagate over. This is the idea of a cosmic boundary. As this boundary moves it can release copious amounts of energy. The released energy can destabilize the boundary further.
In addition, as the wiki article presents, it is possible for a region to have fallen down to one lower energy, but for there to be a still lower energy available.
If a region was in the false vacuum, it might be relatively stable. The barrier to the true vacuum might be quite large. However, a large enough "kick" of the right type might cause the local field to jump to the lower state. This would release enormous amounts of energy. When I say enormous, the difference could well be comparable to nuclear matter densities, and be released through a galactic size volume.
Now, how does this change physical properties? In the false vacuum image, you must imagine that the $\phi$ is really more than one dimension. So the minimum is actually a ring, not a point. There's that Mexican hat again.
The parameters that correspond to the place around the ring are the fundamental constants of physics. Things like charge, mass, etc.
(Falling off that central peak is also one idea of where the matter in the universe came from, and drove the Big Bang. But that is really getting into a lot of very complicated cosmology. It's stuff I'm barely aware of, even with my PhD in particle physics.)
The idea is, the minimum could be anywhere around the ring. The choice of where it goes is randomly selected through a process called "spontaneous symmetry breaking." The specific location determines a lot of things like the ratio of an electron's mass to a proton's mass, the mass of a neutrino (if it's not zero), the ratio of strength of gravitational force to electrical force to strong and weak nuclear force, and so on. If a boundary region moves you from one spot on the ring to another, then these parameters could change. So suddenly, any matter in the region that previously was stable, might suddenly be unstable. Not just chemically but the nucleus in the atoms might fly apart. Or Newton's gravitational constant could change radically.
The result is, if we moved from one region to another, we would be extremely unlikely to survive. Even our atoms would be unlikely to survive.