Chinese proverb

Saw this proverb @ a shop during dinner,

When sitting quietly,
reflect upon one's mistake

When chatting,
do not criticize others

Got the meaning behind this proverb?
If you're, then you're a smart ass! =P
Found it so useful & could teach those bastards out there!
It's a saying from a Chinese Master, couldn't remember his name.
Definitely, written in Chinese words but it comes with English translation too. (: