This is a very powerful idiom and can help us to groom personality when analyzed & understood. It means that we have no right to criticize or take any action for the same kind of mistakes that we commit often, are committed by others.This can be used when we know that the person is blaming us or criticizing us on some of our failures, has been making the same mistakes and failing in those areas.This idiom also helps us to think twice before we criticize or blame others. There are many countries who advocate peace while their own country in a turmoil. Like wise there are bosses in companies who do not observe discipline and take actions on juniors when they break rules.If a person, who himself is committing mistakes and blames others, faces severe reflections when someone bounces back on him / her. The idiom also throws light on `delicacy`, a value of communication skills.Enjoy !