These ratings combined 4 different methods into one standardized rating to help you with identifying the better teams from the bad ones. This is courtesy of the website fivethirtyeight.com as written by Nate Silver.
All of these ratings systems are on different scales. Transfermarket's figures are in dollars or euros, for instance, while Elo ratings are denominated in points. So here they have been standardized to make them comparable to each other.