A false vacuum is an idea from theoretical physics: In quantum field theory, such a vacuum might exist for a very long time, before it changes its state. Much like with a true vacuum, a false vacuum exists where the energy is minimal. In the case of a true vacuum, this is at a global minimum of the energy function. With a false vaccum, the energy function is only at a local minimum. As is illustrated in the figure, there is a "peak" between the local minimum and the global minimum of the energy function.
The process by which the system changes its energy state is called bubble nucleation: Somewhere in the universe, a bubble will appear, with a lower energy state inside the bubble. This bubble will then spread and change the energy state of its surroundings.