Some crimes are unforgivable. Others are venial — venial crimes and sins are excusable. They're not a big deal.
In school, there are so many things that are against the rules: talking during class, tossing spitballs, pulling the fire alarm, stealing a basketball, etc. Something venial would be something against the rules but forgivable. For example, if you were late for school because your parents were in the hospital, that lateness is venial. If you burned the gym down, that could never be considered venial. When you see venial, think "forgivable," "excusable," and "no biggie."