I'm still checking it out, but it is quite interesting thus far.
First, 538 is the number of electors in the electoral college.
Now, from the site's FAQ:
How is this site different from other compilations of polls like Real Clear Politics?
There are several ways that the FiveThityEight methodology differs from other poll compilations. Firstly, we assign each poll a weighting based on that pollster's historical track record, the poll's sample size, and the recentness of the poll. More reliable polls are weighted more heavily in our averages. Secondly, we include a regression estimate based on the demograhics in each state among our 'polls', which helps to account for outlier polls and to stabilize the results. Thirdly, we simulate the election 10,000 times for each site update in order to provide a probabilistic assessment of electoral outcomes.