It's from Nate Silver of FiveThirtyEight fame. It's an aggregate of several polls, each of which are given a weight based on accuracy in the past, as well as a correction based on polling method that could skew the result slightly.

Note that these polls do not correspond to 38% of likely voters, as presidential approval tracking asks a much wider segment of the ~~pipularion~~ population beyond likely voters. But yeah, diligence has been done, as accurate as is possible when it comes to polls. I recommend checking out the link; The methodology is explained there better than I could ever do in a two paragraph comment.