During a match the ball can be free or in possession of one of the teams.
Free Ball
The ball is said to be free when none of the players is located right behind it.
The previous image shows several examples of free balls.
Possession
Teams can gain possession of the ball in two ways. One of them is by placing a player right behind a free ball. The other one is by recovering the ball during a challenge.
The previous animation illustrates both ways. First, a player of the blue team recovers the ball during a challenge; then, it makes a forward pass and the ball is freed, but, finally, another player of the blue team takes it moving right behind it.