If I want to meet someone in New York City, where I live, I might say, "meet me at the corner of 5th ave and 42nd street, on the 10th floor."
Thus, three numbers (labelling height, width, length) allow me to specify any point in the world. However, mystics and mathematicians have always been fascinated by higher dimensions beyond the familiar three. Einstein showed that time can be viewed as a fourth dimension, so that we must also specify the time. But string theory forces us to allow for the existence of 10 dimensions (nine dimensions of space, and one dimension of time).
Any "being" that hypothetically lives in a higher dimension would have the power of a god. Think of looking down on the carp in the pond. Recall that we can make the fish disappear or reappear at will. We can effortlessly move past any walls that exist in the pond. Similarly, a hyper-being might be able to perform feats of "magic," for instance, disappear and reappear, walk through walls, remove gold from a safe without breaking it, perform surgery without cutting the skin, turn right-handed seashells into left-handed seashells, etc.
However, string theorists believe that these dimensions today are too small to harbour life. String theorists believe that at the beginning of time, the universe was a 10 dimensional bubble of some sort that "split" into a four dimensional and a six dimensional universe.
The four dimensional universe expanded, giving us the Big Bang, but the six remaining dimensions collapsed so that they can no longer be seen. Some scientists, however, are convinced that some of these higher dimensions may be the size of a millimetre or so, in which case they can measure their effects in the laboratory.
What lies in a parallel universe?
The parallel universes of the multiverse, for the most part, are probably dead universes. Perhaps the proton is not stable enough to allow for the formation of molecules such as DNA.
Perhaps the nuclear force is so strong that stars burn out too quickly for planets to form. Or perhaps gravity is so strong that a universe pops into existence and dies in a Big Crunch soon afterwards. Many of these parallel universes might consist of a lifeless gas of electrons and neutrinos, which never form stable atoms.
In fact, a remarkable series of "accidents" must occur in a universe to create life. For instance, protons must be stable enough to allow for atoms, molecules must be complex enough to create DNA, stars must live long enough to spawn planets, gravity must be weak enough to allow for an expanding universe but not strong enough to create a Big Crunch, etc etc etc. Scientists have catalogued scores of "accidents" necessary to allow for life in the universe, so life is perhaps extremely rare in the multiverse.
However, it is also conceivable that some of these parallel universes look similar to ours. In fact, perhaps only a single quantum event might separate us from one of these parallel universes.
If a cosmic ray went through Winston Churchill's mother's womb and she suffered a miscarriage, then perhaps Churchill was never born and the Nazis won the Second World War.
Thus, a single quantum event separates us from a universe in which the Nazis dominate the world. Since a single quantum event can also create mutations, then perhaps in some of these worlds humans never separated from the apes, or perhaps they are ruled by super-humans more intelligent than us.
This may not be an academic question. In the far, far distant future, if the universe continues to accelerate, then any future civilisation, facing the Big Freeze, may be forced to contemplate leaving our dying universe and entering a warmer, younger one. The multiverse, once the playground of theoretical physicists, might be the ultimate sanctuary of intelligent life.