The quarterback is the face of the team. He is the captain of the offense. The quarterback is the only player who gets to touch the ball on every single offensive play of the game
When you want to run the ball, the quarterback will hand the ball off the the running back. When you want to pass the ball, the quarterback himself will throw the ball to a receiver.
The quarterback has full control of the play before the play starts, and for that reason he needs to have a big brain. If coach called a play in the huddle thats supposed to beat a certain type of defensive coverage, but you see the defense is running a different type of coverage, its the QBs job to change the play pre-snap (pre-snap just mean 'before he starts the play'. Yeah i also didnt mention the QB is the guy who starts every single play. Like i said - he has full control of the offense pre-snap)
The QB is by far the most valuable player on the team. He needs to have good awareness, good throwing ability, good running ability, and a very high football IQ (aka knowing schemes and coverages and blocking etc. very in depth). This is a lot to ask though and its why so many guys dont make it as a qb.
This is an oversimplification but hopefully it gets the point across