Okay, here's the thing. There is no fourth dimension. Mind not blown by that? How about this - there is no third dimension either. Before you start yelling at your smartphone about how the Universe is made up of three dimensions, just settle down and let the episode of MinutePhysics above explain this rather tricky misconception.
Anyone with a basic understanding of dimensions could tell you that an object with height, width, and depth is three-dimensional, and something with height, width, but no depth - say a drawing on a piece of paper - is two-dimensional.
The problem is that these are characteristics we can measure in two- and three-dimensional objects, but they're not dimensions per se. In reality, we can't actually tell the three spatial dimensions of an object apart.
Henry from MinutePhysics explains what this means by giving you a jug with three cups of water in it, and then asking you which is the first cup. No idea? How about you point to the second cup, or the third one. Wha??
Yep, those questions don't even make sense, but they do if you add a cup of oil to the mix, because oil behaves differently to water. Throw in your oil, and now you have three dimensions that behave in the same way (the three cups of water that you can't tell apart) and one that behaves in another way (that one cup of oil).
But just as you can keep adding more cups of water to your hypothetical jug, we could keep adding more than three spatial dimensions to our one time dimension in the Universe.
That's what string theory is all about - physicists have used it to explain how the Universe could be made up of as many as 11 spatial dimensions, plus one time dimension, even if humans can only perceive three.
Watch the episode of MinutePhysics above to wrap your head around this mind-bender, and what Henry try to solve a four-dimensional Rubik's Cube. What even is life.