This statement is warning the danger of prejudice.
Every moment in which we face with others, we judge them by a lot of factors; appearance, race, sex, education background, experience, and so on. Those factors make people prejudice against others.
In my opinion, prejudicing itself is not bad. The important thing is how we react to our prejudice or convert ideas into action. For example, if it were developed to the discrimination, it can make problems. On the other hand, if we don't discriminate against others even though we already prejudice them, there is no problem.
In this world, discriminating against someone or specific group such as the black or the fat can be detrimental to one's own reputation because discrimination is sort of the absurd. Nowadays, whether there have been changed on people's mind, visible discrimination is decreased. I don't know whether this trend is good or bad.
What I want to point out is that discrimination over prejudice will eventually come back to themselves in any form.