It says how often you are sacked each time you face pressure.
Barnes rarely gets pressure (he has loads of time to throw), and then when he gets pressured he's rarely sacked (only 10% of the time). Barnes is pretty good at getting rid of the ball based on my observation.
On the left side of the X axis the QBs get rid of the ball very quickly (either because they are pressured often or else it is part of their scheme).
On the top of the Y axis the QB isn't good at getting rid of the ball when pressured.
Milroe is giving up a sack 40% of the time he is pressured. Just my observation from watching Bama games, but receivers don't tend to get open for Milroe and then he holds onto the ball too long.