After your defense makes a stop and regains possession of the ball, your team will begin to clear the ball.
In a lacrosse game, clearing means to get the ball from the defensive side of the field to the offensive side of the field. A great clear can lead to a fast break goal. You and your team will soon notice if you are making failed clears—however, in the intangibles of momentum and morale, and also practically—because without successful clears you will be playing a lot of defense and will not have the ball in position to attack the opponent's goal and score.
In sports such as basketball, hockey, and soccer, getting the ball from one side to the other is fairly automatic outside of a few special scenarios. In lacrosse, it is anything but automatic and can be a place for a team to lose a game or turn the tide in their favor.