Sure, I'll give it a try. If you put two bar magnets next to each other, they tend to flip each other around so that they point in the same direction. Now try to picture an infinitely large universe, which is filled with an infinite number of tiny bar magnets. If all of these magnets pointed in the same direction, there wouldn't be much interesting going on; since all the tiny magnets are already aligned, they won't try to flip each other over, and the universe would be a stable place. (You could still have some fun by flipping a few magnets, and watching the ripples spread as a wave throughout the universe; but that's not what I'm gonna talk about now.)
But let us now consider a different scenario: in one end of the universe, all the magnets are pointing "up", while in the other end of the universe, all the magnets are pointing "down". By themselves, both these regions are stable, since there is nothing inherently "better" about pointing up than pointing down. However, somewhere in between these two far ends of the universe, there has to be a region where the magnets change from pointing up to pointing down; and this is a region of higher energy, since you have all these tiny magnets which are constantly fighting among themselves about which way to point, and constantly trying to flip each other over. This is called a "domain wall" in the case of magnetism, which is an example of a "topological defect". This domain wall can be moved and twisted by flipping a finite number of magnets in the vicinty of the domain wall; but you can't truly get rid of it without flipping an infinite number of magnets throughout the universe, which would end up requiring an infinite amount of energy.
In some quantum field theories, you get analogous situations where a theory has multiple stable "vacuum solutions". If the universe contains fields like that, we would then have two possible scenarios: (i) the entire universe has the same vacuum state (corresponding to all the magnets pointing in the same direction); or (ii) the universe could in principle consist of different stable regions with different vacuum states, with an unstable region called a "topological defect" inbetween, where the different vacua fight for dominance.