Passing the ball in NFL matches is very important and even more important is to complete the pass. The pass is completed when the ball is caught by wide receiver or the right end. If however the ball touches the ground first before the players from the offense team is able to catch it, the pass is considered incomplete.
As for the defense team playing in the NFL match they can prevent the opponents from advancing the ball and gaining yardage by bringing the player carrying the ball to the ground. This technique is called as tackling and when the knee of the player carrying the ball touches the ground he is tackled. In such circumstances the play is over. Another way of ending the play is when a player runs off bounds. Like in any other sport, the main object of the NFL matches is to score the highest points. There are 4 ways of scoring points. One is touchdown which gives six points, the extra point and 2-point conversion gives one to two points, field goal gives 3 points and safety gives 2 points. The teams have to score as many points as possible to remain in the lead.