Back in 2012 jeffdos24 had the easiest to understand explanation I've seen.
"... That light blue area on either side of the ding line represent your margin of error .. the farther you miss the ding, the more off-line your shot will be. In real life, the swing is a pretty fluid motion -- smooth back swing, slight pause at the top, then the downswing. Here in WGT land, you'll notice that that light blue zone doesn't start shrinking until you stop your backswing. When that zone shrinks, so does that margin of error. A 1 or 2 pixel miss isn't bad at all when you have a full blue area. Hold at the top of your swing and let that zone shink to almost nil, and that same 1 or 2 pixels will go a mile off line."