To many people, the quarterback is the indispensable leader of the team. A quarterback is important, true; however, there is some question about just how important he is. Certainly, if the quarterback is having a bad day, there is a great chance that the team will lose, unless the other team is having an even poorer one. However, there is a statement that has been made many times that may be more to the point: A quarterback is only as good as his offensive line.
Quarterbacks have a lot of duties. They must know the various plays, at times a large number of them, and how they are supposed to be run correctly. He must also be able to relay that information to his teammates so they understand their role in each play. They are expected to be able to hand the ball off, to have some mobility and to be able to pass the football accurately; all of this while being able to correctly read the defense so they have an idea of what the defensive players are going to do. Even this is a simplified rundown of their job duties. A quarterback is also often the cheerleader in the huddle, encouraging people yet chastising them when necessary.
Mobile quarterbacks are those able to move around to avoid rushes. However, the best quarterbacks are mobile to the point where they can also run effectively, in essence, giving the team an extra running back. If they can both run and throw accurately, they can become potent weapons for their teams, provided that they have offensive line support.
Offensive line duties
In theory, the offensive line has much simpler duties, though in practice it isn't quite that easy. They are supposed to block the defense they are playing against. The purpose is to prevent the quarterback or running back from getting tackled, especially behind the line of scrimmage. Offensive linemen are also supposed to open throwing lanes or open holes in the defensive lines for a runner to get through. A lot of this is done by pushing the defensive man back, turning them to the side or at least keeping them in one position and not allowing them to advance, though there are exceptions such as in screen pass plays.
Ends also block but they aren't being included here because they are also supposed to catch the ball and to block down field. By offensive line, the focus is primarily on two tackles, two guards and the center.
Importance of the line
If the line doesn't function as it is supposed to, the frequent result is that the defense blocks lanes, blocks attempted passes, tackles the running back for little or no gain or they sack the quarterback. None of this is positive and it can lead to worse, such as fumbles and interceptions. Often, this isn't because a single person made a mistake though, since it is a team effort.
It should be considered that even a great passing quarterback will have some difficulties passing the ball when he is flat on his back before he can get rid of it. He might be mobile and even a terrific runner, and yet without a bit of help from the offensive line, to stop or slow down the defensive players, the man is usually going to go down.
Thus, the quarterback is only as good as his offensive line. This goes a long way toward also explaining why some quarterbacks who've demonstrated repeatedly that they are among the best quarterbacks in the league can end up with a loss so bad that it is embarrassing. At the same time, it explains why some quarterbacks who have not been much more than mediocre can have a game in which they seem to be brilliant. A good offensive line can make a world of a difference in either case.