I'm horrible at explaining things
The farther left and down the teams are on the graph the more effective their defenses have been.
The team with the blue and green bird (Seahawks) has one of the defenses in the league. They are very good at keeping the opposing team's offense from running or passing the ball down the field thus keeping that team from gaining yards (unit of measurement) during their plays and keeping them from scoring.
They allow the least amount of points to be scored against them per game than any other defense in the league.