Harsh means severe or strict, particularly when referring to punishment. It would be a harsh punishment if you got grounded for two weeks just for coming home a little late for dinner.
There are many shades of meaning for the word harsh depending on its context, but all imply that something is more unpleasant than it needs to be. Harsh lighting hurts your eyes. A harsh disciplinarian gives out punishments that are too severe. Whereas constructive criticism is usually helpful, a harsh critique will only make you upset. The harsh reality of the world today is that millions of people are starving while others have too much.