It's a stronger and more formal word than "scold" or "reprimand." It implies a sense of anger and disappointment, and often involves a desire to correct someone's behavior.
Here are some examples:
* The teacher chastised the students for their disruptive behavior.
* The coach chastised the team for their lack of effort.
* The parents chastised their child for lying.
You can also use "chastise" in a more metaphorical sense to mean to criticize harshly:
* The critic chastised the movie for its unrealistic plot.
* The editorial chastised the government for its lack of action.