E8 is a shape that exists in nature, and was discovered along time ago, in 1887 I believe they said. However, at that time, and up until just recently, they have not been able to figure out the shape mathematically. Finally, after many years of work  they have been able to mathematically understand the E8 structure. At least that is my understanding of it.
Take for example, a straight line across your piece of paper. That line can be mathematically represented by some 2-dimensional equation along the lines of y=x. That is a mathematical representation of the figure you are viewing.
Now compare that to E8. E8 is a a similar idea, however, instead of having the 2-dimensional line on your paper, it is a 248-dimensional figure existing in free space. How do you mathematically represent that figure? It's obviously much much more difficult. That is what they just found out.
With the E8 idea in mind, a physicist began formulating laws of the universe and calculating all the different forces and such that interact among one another. With the new mathematical representation of the E8 structure, he found that his versions of the laws of physics fit into the E8 formula. I think that is how it works, however I am kind of fuzzy on that aspect. What is great about his versions of the laws of physics is that they cover only 3 dimensions, and a 4th, time, and they all fit perfectly within this E8 structure. Other theories that physicists have believed to be true (i.e. string theory) are much more complicated, ranging over 16 dimensions I believe, and they don't know how to produce any experiments to test these theories. For this new theory though, being only over 3 dimensions and time it is much easier, seeing as how everything we do is based in 3 dimensions and time.
That is my understanding of it at least. Correct me if I'm wrong in any of it.