Here’s a bit of advanced and really complicated quantum physics theory made a little bit more simply.
Einstein’s famous equation, E=MC2, demonstrated that energy and matter are essentially related and one can be converted into the other. Einstein’s theory of general relativity, however, doesn’t properly describe the behaviors of subatomic particles and new observations in physics required the development of a new theory of matter.
Theories of matter keep looking toward smaller scales. In the past, atoms were thought to be the smallest particles possible. However, subatomic particles called neutrons, protons, and electrons were theorized and then observed through their different behaviors. Further subdividing, sub-atomic particles have been observed to consist of even smaller particles called elementary particles, such as quarks which make up neutrons.
String theory is a different type of theory of matter which moves away from point-like particles and represents matter as being constructed of incredibly short ‘strings’ which change orientation, meet, interact, vibrate, and carry direction and is this behavior that produces the effects that earlier researchers believed were proof of individual particles. So in this theory, everything is made of incredibly small strings. So far, this theory of matter is just a theory as no one has ever managed to observe a string or prove its behavior, however many physicists believe that the theory is mathematically correct and is close to explain all of the phenomena that science has managed to observe to date.