A down is a try. A team gets 4 tries or downs to move the football 10 yards. if they can't move it 10 yards they have to give the ball to the other team. Usually on 4th down (the 4th try) the team will kick the ball away. This means they kick the ball as far down the field as they possibly can so the opposing team has start start further back. Sometimes on 4th down they will "go for it" this means they are going to try on their last down to get that 10 yards. If they don't get it, they have to give the ball to the opposing team right where it is on the end of the 4th down.