What is that?
Batting average on balls in play. It wouldn't actually be negative. Most people believe it at least in part is an indicator of a hitter's luck, though it's influenced by things like line drive/flyout/groundball rate. A high BABIP would indicate bloop hits and lucky hits falling. A low one could indicate making contact but hitting it at people.
A high BABIP inflates your average. So you look at a guy like LaRoche, his career BABIP is around .300(average) but right now it's .212.