Man 1: Just what is string theory? What are these strings?
Man 2: Er, string theory is an attempt to make sense about all the phenomenon we see in nature in terms of one unified field theory.
So, 20th century physics basically understood reality, or as much as we know of reality, in terms of two big theories that unites the quantum mechanics that describes the atoms and molecules, with general relativity which describes the stars, planets and galaxies.
And, of course we have to unite them somehow, because the stars, galaxies and planets are ultimately made out of atoms.
So it wouldn't make sense to have one theory that only works for atoms, while another theory entirely would be used for stars and galaxies.
They're all made out of the same stuff, so they have to be described by the same theory.
