String theory is a theoretical framework in physics that attempts to reconcile quantum mechanics and general relativity. It proposes that the fundamental building blocks of the universe are not point-like particles, but rather tiny, one-dimensional "strings" that vibrate at different frequencies. These vibrations give rise to the different particles and forces in the universe. String theory also suggests the existence of extra dimensions beyond the three spatial dimensions we are familiar with. However, it is still a highly speculative and controversial theory, and there is currently no experimental evidence to support it.