FiveThirtyEight is a bit of a Presidential polling mash-up. I have a love/hate think with polls. I think they are meaningless this far away for election day. They emphasize the horse race aspect while folks show vote for who they believe will be the best for the country. They often are biased in the way they ask questions and their numbers can be manipulated. As we have seen in the past, "margin of error" is a meaningless phrase. I also love numbers.
FiveThirtyEight tried to take all of the polls out there, mix them up and come up with something that might be closer to the truth. This has some scientific basis. (I recently read a story about how if you have a 100 people guessing the number of jellybeans in a jar, there is a good chance no one will get it right. BUT if you take all of their guesses, their average (or median... I forget) will be darn close.) They also give different weight to different polls based on past history and other factors. It's interesting stuff.