Reactions in our universe tend towards moving things from less stable states to more stable states. When they do this they can release energy. Often you need to put a little energy in to get something to move to a lower energy state though - eg you have a candle - you have to light a match and then light the candle before it starts processing hydrocarbons and oxygen into CO2 and H2O - but once it does it’ll release a lot more energy than you needed to start it off.

All known reactions in the universe take place within an assumed baseline - everything happens relative to that. False Vacuum Decay is saying “but what if that’s not really the baseline”.

Eg:

Imagine you have a bowl of marbles. They’re all trying to be near the bottom. You can shake the bowl around a fair bit and they’ll move and then return to the bottom. This is the assumed baseline. False vacuum is saying “but what is that bowl is on a table - if you shake it enough a marble can fall out and fall to the floor”.

The consequences if this is the case and if something does put enough energy into something to cause it to go over the edge is that the reaction could release enough energy to drag everything else over the edge in a rather spectacular, albeit destructive, way.