EPA tiers plot teams by their offensive (x-axis) and defensive (y-axis) performance as measured by EPA / Play.
The grey tier lines create comparisons across teams. Teams on the same line are "equal" and expected to perform similarly on a forward looking basis. Because offensive performance is more predictive than defensive performance, these lines have a slope greater than 1 (1.6).
The tiers themselves represent the historical distribution of teams against the league average (median). For instance, a team at the "p90" line would be better than 90% of teams in any given year.