Ordination procedure in Theravada Buddhism - What are grievous crimes?

I read in this answer that if one wants to become a Bhikkhu then *"He must never have committed any grievous crimes"*. - What could examples of grievous crimes be? - What if crimes where committed many years ago. Does that have any value in the ordination procedure or does it automatically result in one not being able to become ordained in this life? Thank you for your time.