String Theories are theoretical frameworks which try to describe various observed phenomena in quantum physics and cosmology.

They first originated in the 1960s as an attempt to understand unexplained aspects of the Strong Force which holds atomic nuclei together.

Since then, they have branched-out to cover a multitude of physics problems - quantum gravity, nuclear theory, particle physics etc etc.

The core premise is that atomic 'particles' (electrons, quarks etc etc ) can be thought of not as a 'particles' as such, but rather as infinitesimally small dynamic 'strings' - one-dimensional entities in a multi-dimensional universe. The number of 'dimensions' required for the associated mathematics to make sense varies according to the branch of string theory - but can range from 10 to over 25 (including the 3 known physical dimensions plus 'time')

Proponents of the theories assert that they can provide plausible explanations for quantum gravity, condensed matter physics &etc.

However . . .

The fact that we only have access to 3 dimensions (plus time) means that no experiments can be devised to test the theories. In addition, the astonishingly small size of the proposed strings (10⁻³⁵ meters or so *) would require unimaginably powerful and sensitive particle-accelerator technology to test even the most basic theories.

There is no experimental evidence that string theory is the correct description of our world and scant hope that hard evidence will arise in the near future. Moreover, string theory is very much a work in progress and certain aspects of the theory are far from understood. Unresolved issues abound and it seems likely that the final formulation has yet to be written.

Source : String Theory, University of Cambridge. Part III, Mathematical Tripos

For those reasons, in Wikenigma, String Theory is currently tagged 'Unfalsifiable' .

For a comprehensive technical guide to the history and current progress in String Theory, see the link above, and also Wikipedia

* Note: To put the predicted string size into perspective, 10⁻³⁵ meters is roughly 5 million trillion times smaller than the diameter of a proton.