The string theory is a theory that tries to unify all the forces of the standard model . This theory is consistent, but nothing it predicts has been discovered yet.
The string theory is based on the idea that all particles found in the standard model are actually a string. This string is similar to a guitar or violin string, and depending on how it vibrates, it is seen as a different particle with different mass and energy.
About String Theory in German. (Stringtheorie)