Dear Sehic,

Thanks for the question.  

Grave or serious matter typically involves something that does significant harm to another, oneself, or against God (though all offenses are ultimately against Him).  Some examples of this would include adultery, other sexual sins, criminal theft, murder and assaults, blasphemy, sacrilege, etc.  We tend to have an intuition regarding some of these things.  A "white lie" may have little effect but slander and bearing false witness can do serious injury.  The former sin is venial, the second more likely mortal sin.

Our own justice system reflects a similar assessment of the gravity of offenses.  We attempt to sort out the following:

• the scale of the crime
• the foreknowledge and purposefulness
•  the immediate circumstances, and
• the knowledge of the perpetrator.  

This range of culpability is always taken into consideration when we try to determine the culpability and punishment for the offense.  There is no doubt that the criminal justice system has taken its cue from the values inherent in Western Civilization, raised in Christendom, the fruit of the Catholic Faith.  
Peace,