When a long ball is played into the opponents defensive players and you aren't able to win the first ball (meaning, the opponent will get to the ball before you will), many players will realize that they can try to win the second ball.  This simply means conceding that the opponent will make the first touch of the ball so you try to take advantage of what might be a bad first touch and you win the second touch.

In the example below, the red player plays a long ball because they are under pressure and it goes toward the red central defender.  A yellow forward is laying back with the idea that they will be able to win the second ball.

The problem with this is that the yellow central defender isn't being put under any pressure at all and as a result of this, most likely will be able to receive the ball with the first touch and keep it under control and away from the red player.

In order to win the second touch, the first touch usually needs to be pressured.  When possible, this would be done with a second player.  In the diagram below, the same ball is played in but now, instead of the red player staying back and hoping to win a second touch, he pressures the player and a second player on his team comes in to win the bad touch.

By pressuring the ball, it changes things from hoping for a bad touch to creating the bad touch by the opponent. This greatly increases the likelihood of you, or your teammate, winning the ball and creating a goal scoring opportunity.