Well, that depends who you ask: string theory is still very far from being able to make conclusive claims about our universe (and, of course, it could just be wrong!). But a common model in string theory for our universe is what's called the brane-world model. In this model, our universe is actually just a four- dimensional (three space plus one time) membrane (or brane) that exists in a higher-dimensional (ten or eleven dimensions!) space. The reason this model is popular is because it gives a possible explanation for why gravity is the weakest of the four fundamental forces: evidently, the electromagnetic, weak, and strong forces are confined to only act between matter on our brane. However, gravity is free to "escape" from the brane, and it's this "leaking" which makes gravity much weaker than the other three forces.

While that's one of the prevailing ideas, it's important to remember that it's far from certain, and it will be a while before string theory can make definitive claims like this!

There are still many things about string theory that we don't including how, if it all, string theory describes the world we live in. However, one theory is that we are living on a 3- dimensional object called a 'brane' which is a part of a 9-dimensional space. (Maybe it looks something like this

One reason some people like this idea is it might explain why gravity is so much weaker than the other fundamental forces (like electromagnetism) but so far there is no concrete evidence for or against the idea that we live on a brane.