Ground balls equal possessions Basically, its picking up the ball off the ground following a face off or a turnover (including when you dislodge the ball from you opponent) or basically any loose ball situation. Picking up the ball after a goalie blocks a shot is a ground ball too.
Mainly due to face offs, there are much more loose ball situations in lacrosse than any other sports. The puck is loose quite often in Hockey, but it is not an apples to apples comparison. There are very few loose balls in basketball and fumbles in a football game.
Particularly when kids are younger, the team that wins the ground ball battle wins the game. In college, its plausible to lose the ground ball battle but win the game.