In this case, movie ratings from three sources (IMDb, Rotten Tomatoes, and Metacritic). To avoid the necessity of a third z-axis, one set of data is presented using a color code.
The graph itself has little useful information except to display a general concordance; the identification of selected outliers serves no useful purpose. Posted to illustrate the concept of displaying a third variable using color.
Discussed at the Dataisbeautiful subreddit.