The Same Ingredient
In String Theory, everything in the universe is made of really tiny strings, all the same except for the way that they move. In other words, everything is made of the exact same ingredient. That would mean the only difference in these "ingredients" is the frequency at which they vibrate. Crazy, right?
Even crazier, if this theory were to be true, then the math would suggest that there are 10 or 11 dimensions!
But what gravity is a massless particle? These tiny strings could explain how we could unify General Relativity and Quantum Mechanics. By applying this theory to complicated math, the answers end up matching, instead of there being anomalies.
Below is a video that gives a clear explanation of String Theory. It may seem like a lot of information at once, but don't let it scare you!
Science or Philosophy?
There is much controversy over whether or not String Theory should be considered science. The nature of this theory is questioned because there is absolutely no way to test it.