dxb, if you combine my if a team advances as much as ten yards (a football field is 100 yards in length between the end zones), then they get a "first down"--thus, if they get enough first downs, they can get close enough to the end zone that, when the quarterback receives the ball, he may just be able to jump over the line of scrimmage and land in the end zone (not that this is an easy maneuver, mind); however--if they have NOT gained ten yards after the third down, most of the time their next play will be to kick the ball downfield as far toward their end zone as possible; this means that they are acknowledging that they had to turn the ball over to the opponents, but they want to make sure the opponents have to start as far away as possible from their end zone, which is behind the team that just gave up the ball. Occasionally, a team will do a regular play on a 4th down; usually in extenuating circumstances such as being only a short distance from scoring a touchdown.
with your quote
DOWN: An offensive play, starting with a center snap and ending when the ball is dead. The offense gets four downs to gain ten yards. If they gain that before using all four downs, they get a first down and another four downs to gain another ten yards. Each time they gain ten yards, the team is at their first down again, needing another ten yards(first and ten) within the next four downs, or plays. , then perhaps the overall picture will be clearer.
I should add that yardage is gained either by a player running with the ball, or by passing (throwing) it successfully to a teammate who is further downfield.
