There is a problem in physics: we have these two very successful branches, one is called quantum mechanics and the other general relativity. While they both work in their respective field, it’s hard to make them work with each other as part of general theory to explain the forces in nature (they all this “Theory of Everything” – physicists are a bit lazy when thinking names).
String theory is one contender proposed intended to mix quantum mechanics and general relativity. The main idea is that matter, at a length scale which is smaller than one atom (so less than one billion of a metre), is composed from tiny vibrating strings of energy.
While it is consistent with the two theories, string theory has been attracting much criticism because it is all theoretical at the moment. We cannot make experiments to prove that these strings are actually there or not!
It’s up to the next generation of scientists like yourself to think of ways we can “probe” these tiny strings, to see if they exists or not, and find out a way to explain the physics in our universe.
One of the leading scientists in the world, Professor Brian Green has published very nice books about string theory. You can even find him talking about string theory on youtube! He surely knows a lot better about the subject than me.