"It is better..." If people believe that you are a stupid person, then you should say nothing. If you say something, people will think that you are even more stupid than before.

If you are a fool, keep quiet. People may think you are a fool, but they may not be sure. There remains a little doubt perhaps. If you are a fool, and open your mouth and say something stupid..... oh dear! People will now know you are a fool for certain. It's a nice little saying. It has some wisdom in it! I think it is also a general reminder to think before you speak! Better to be quiet and think about something (even if people assume you a being a bit slow/stupid) than to shout out the first thing that comes into your head (which is probably not a very good idea).

宁可保持沉默，即使这样会让人觉得自己是个傻瓜，也不要出声说话，让别人因此对你不再有疑虑。