Fly ball precentage refers to the percentage of balls in play that are fly balls. Although this statistic is more closely monitored for pitchers, it applies to batters as well. A pitcher with a low fly ball percentage will normally be more successful than one with a higher percentage, since home runs are typically hit on fly balls.
Also known as "fly ball rate".
For a pitcher, a low fly ball percentage means being able to keep the ball in the ballpark. Limiting hitters to line drives and ground balls means they must work a lot harder to manufacture runs. For a batter, fly balls mean possible homers. After all, it's a little difficult to hit a home run on a ground ball, isn't it? On average, 10 percent of all fly balls leave the ballpark, so if a batter is only getting the ball in the air at a 10-20% clip, then he is not giving himself much of a chance for power numbers.