At the line, the QB is calling out the defensive positioning a lot of time (so the rest of his team knows what defense is being run), as well as distracting the defense on when they are going to snap the ball

[removed]

Everyone is talking to each other , qb is calling out any changes , linemen are calling out blocking assignments, rb will step forward to hear or say something to qb if he sees something

[removed]

The QB calls an audible when he believes changing the play will lead to a better outcome than running the huddle play.

There are a huge number of defensive formations and offensive plays built to exploit each ones weakness, so if changing a play can allow taking advantage of a better matchup than the called play, many coach's systems allow the QB to make a read and change the play.

The other stuff is there to communicate various sub play options, like how a blitzing player should be blocked, and then there's a ton of chatter designed to prevent the defense from predicting the snap count.

Sometimes the QB will call two plays in the huddle. Then at the line the QB will make an audible to tell the line which one to do depending on how he read the defense and if he sees a weak spot for the receiver to exploit.