The string theory is a very interesting idea that propose another totally different way of thinking what made up our universe. Although this theory is very arguable because it can never be tested in our labs so that a lot of physicist do not like this theory, but it indeed gives a very unique prospective and interpretation about parallel universe and dimensions.
The theory proposes that everything in the universe is made of the tiny element called string. It is itself one dimension particle. String theory proposes a unique solution trying to figure out the ultimate equation of the universe, that equation that unifies all four major forces in the universe: gravity, strong nuclear force, weak nuclear force and electromagnetic. This is the equation that Einstein seemed for his whole life.
Before string theory, people think this equation does not exist because of the quantum physics. Modern quantum physics thinks that things happen in the quantum field can not be predicted. We can only predict the possibility of something happen in the quantum field. In other words, there are no equations related to quantum physics since everything is about probability.
Einstein believes that there must be an ultimate equation that explain everything from large scale to quantum physics. String theory provides one possible answer for seeking this equation. String theory proposes that every forces are generated because of specific particles. For example, the one that is responsible for gravity, also the special one, is called graviton. This is a theoretic particle that has never been found by human.
Other things that are very interesting about string theory is that it tries to explain the parallel universe by the idea of membrane. Basically our universe is like a membrane where there are a lot of other parallel universe. Only graviton can sometimes travel through membranes.