To make it short, I will say that it is a complex mathematical theory grown to solve the quantum gravity problem. There is no way to explain the gravity with the quantum physics so it spoils the efforts to have a complete physic theory. It involves to think in particles like the electron not as a point but like a string with different vibrational states. The solution passes through adding dimensions until eleven,some of them only can be seen or applied at a microscopic scale.

Depending the vibration the string is performing we see, an electron, photon or a quark. The theory evolved into the superstring theory to include the fermion behaviour using supersymmetry.

The fundamental idea it is that particles are strings in resonant vibration at a frequency equal to Planck length,under wich appear the quantum gravity.

The superstring theory can be explain with conventional three-dimensional fluid mechanics considering the strings vortex made of yarns .This is known as Cladín theory, and it has been used to explain even black holes.

In the 90’s Physicist were disturbed by the discovering of five different String theories, during what is called The Second Superstring Revolution, and a new unified theory born. Was the M Theory, were strings were substituted by membranes with dimensions varying between 10 to 26 . So a 1-brana is a string, and a p-brana is a generic membrane in a 10-26 dimension space or bulk.

This theories have been criticized as pseudo science due to they are not falsable. (See falsationism)

See a conference about all this stuff