String theory is called the theory of everything. But why? The reason is very interesting behind it. We have mainly visible two parts of physics, one is classical physics and another one is quantum physics . Almost all theories of classical physics can be described using general & special relativity because they are relative .Relative means they need a frame of reference to describe .And in quantum physics , theories are described through subatomic conjunctions, like-quantum mechanism, describing superpositions ,quantum entanglement etc. They are not always relative . If we can relate relativity with quantum, that will be THE THEORY OF EVERYTHING.
We know that there are four fundamental forces in the universe. They are- gravity, strong nuclear force, weak nuclear force, electromagnetic force. Among them, the other three forces apart form gravity can be described using general relativity. There is a function of particles behind every forces.But scientists are not yet sure for which particles gravity works.Then they go through a hypothesis like this - String is the smallest part of atoms. It is so small that we can’t imagine.String is of two types- opened & closed string. Imagine string is alike a fibre (সুতা). When it is closed, the two edges of the string will be tied & when it is opened, the edges will be untied.Gravity works for a point particle named graviton.Now we’ll go to hyperdimensional universe to find gravitons.
Suppose our observable universe is a slice like bread of the 10 dimensional universe. Graviton is produced in the hyperdimension and affects as gravity in our 3 dimensional universe. Gravitons are unstable particles. They contains closed strings in their quarks.
Imagine the higgs field around us like a polo board & the balls on it like particles.Graviton, being produced from another universe, will fall in this polo board and vanish within a few moments. As gravitons have closed strings, they must not stuck with the board. As a result, they vanish. This is considered as the major reason for gravity being such weak than electromagnetic force.(when we comb our hair, almost all of us have done this experiment that a small piece of paper is lifted up by the electromagnetic force gathered in the claw. Remind that the gravitational force of the whole earth is acting on the paper at the same time. But a little magnetic force is enough to beat up the gravity of the earth. Hope now you can understand why gravity is so weak).
This is the main concept of string theory, but recollect that it is only an believable assumption of the scientists. We are not yet sure that the string theory is 100 correct.