You have a ball on a trampoline. Wind might knock the ball around, but its very likely to return to the center of the trampoline. This is a psuedo stable arrangment.

Now imagine that someone kicks the ball, or the wind blows much harder than normal. The ball might roll up the sides of the trampoline, and then roll off the side onto the ground. This is a truly stable arrangment(eithin the bounds of this metaphor)

Now imagine that there is an invisible field all throughout the universe, and that everywhere along it there is some amount of energy. Because this energy is roughly the same everywhere, there is no gradient to take advantage of, and therefore this field does no work.(Think trying to roll a ball around on a flat surface, the only way to do that is for you to put energy in)

But someone or something decides to inject a ton of energy into a very small space on this field. That allows the field(or ball) to have the energy required to randomly fall into a more stable state(fall off the side of the trampoline)

The issue is that during the process of falling into a more stable state, energy is released. Such as the ball falling off the side of the trampoline, you can harness that energy of motion. But in the case of the field, such a large amount of energy is released that it can cause other, nearby, bits of the field to fall into a more stable state. This causes a chain reaction where the entire field starts to “decay” into a more steady and stable state. This would expand outwards at the speed of energy(the speed of light, or causality) and envelop the entire universe within its sphere or influence(the observable universe from the spot where the reaction started, kinda)

That is vacuum decay. Where what we think of as a mostly empty vacuum of space suddenly decays into a much closer to empty state. But reality and physics as we know them are built upon this semi stable field, and its removal could be massively detrimental(think the destruction of everything at a subatomic level) or it could not do anything at all. The latter is much more unlikely because something had to interact with that field to get it to decay, and if something interacts with it, its likely most everything interacts with it.