In association football, a red card is shown by a referee to signify what?

I played up to the semi professional level here in the U.S. until an injury ended my career in the early 80's, coached for 30 years and also was a referee. I only received a red card once, and it was for asking a referee "how much money did you take from the other team"? never for a vicious foul....

Pierluigi Collina! Boss Man!