There are two imaginary lines in rugby: The gain line and the tackle line. The gain line extends horizontally across the field and is where the ball starts from either a set piece (scrum, line out, free kick, penalty) or a breakdown (ruck or maul).
A team in attack that crosses the gain line will have players running on to the ball, whereas a team which is stopped before the gain line will have players running back. Getting over the gain line is therefore a key aim.
However, to get over the gain line, you need to break the tackle line. This is a line of defenders which is moving forward as the ball is passed out.
If the defence is organised, an attacking team needs to get across tackle line by using good footwork and well-planned set moves. Set moves only get an attacking team over the gain line if every player knows the call and the set move has been rehearsed meticulously (practiced to perfection) at training. It is therefore critical that every player is present at training each week and is familiar with the set play, the call, and his role in the set play. When set plays fail it is usually because they were not rehearsed enough at training, or a player was unfamiliar with the play or misheard the call.