What is string theory?

String theory is a theoretical framework that is yet to be proven. This is a framework that attempts to unify the four fundamental forces known to us: electromagnetic force, gravitational force, strong nuclear force, and weak nuclear force.
One of the main assumptions of string theory is that all particles can be thought of as small vibrating strings and that each of the vibrations corresponds to a different particle. In other words, if the string oscillates in a particular way, it can be an electron. If the oscillations are different, the string can represent another fundamental particle.
Another interesting aspect of the theory is the assumption of additional dimensions. We are familiar with four dimensions corresponding to the three dimensions of space and time. String theory requires 7 more dimensions in addition to the 4 conventional dimensions.
Hope this helps.