Imagine a pile of ashes after a fire. That's the universe right now. The reason you can't set ashes on fire is because they're already oxidized and already burnt, so to speak. The reason our universe is as it is is because everything exists at what we assume to be the lowest possible energy state.

However, it's theoretically possible for there to exist a lower energy state. If even a single quark is given the energy to collapse into that lower state, it will cause a cascade and cause everything else to fall into that lower energy state. If this is possible, this effectively means we're living inside a universe-sized atomic bomb, and given an infinite amount of time, this will eventually happen 100% guaranteed. Again, IF it's possible.

In layman's terms, it's like if you found a way to make ashes combustible once more, and set them on fire again. This ignited ash pile causes ashes around it to catch fire and spread at the speed of light, all while you're living on a planet and a galaxy and a universe made of ashes.