String theory is the most well known candidate for a theory of everything — a mathematical framework that would meld the world of the very small, described by quantum mechanics, and the very large, as described by Albert Einstein's general theory of relativity.
So far, these two theories do not agree with each other, and the problem comes from gravity. In an attempt to integrate gravity (which is weak at small scales where the other three fundamental forces are strong) string theory postulates that the universe is made up of tiny one-dimensional strings whose vibrations produce the particles we see.
