You’ve probably heard of the term string theory one way or another. Perhaps on a science program; or a sitcom.
String theory is the idea that one-dimensional objects called strings tie together all of the universe’s particles and interactions.
So rather than having four separate fundamental forces (i.e. strong, gravity, weak, and electromagnetism), there’s just one unifying theory that covers all of them.
String theory explained
To give you a better idea, here’s a relatively quick overview of how the theory works