Better to remain silent and be thought a fool than to speak out and remove all doubt. If you open your mouth and do not know what you are talking about everyone will know that you are a fool. A wise man keeps his counsel unless he has something enlightening or important to say. Those who are typically quiet are listening and absorbing in everything that they see and hear, gaining knowledge while also being seen as someone smart enough not to talk until they have knowledge of the topic. If you can not add something new and interesting to the conversation then you are better off staying silent and hearing what others have to say instead. If you speak out and you are a fool everyone will realize this fact as soon as you open your mouth.
If others think you are a fool because you are silent that is okay, let them underestimate you at their own peril. When the time comes for you to actually say something these individuals will realize their mistake and feel foolish themselves. It is usually better to do this then it is to speak up and talk and confirm what people think so that there is no doubt at all. Only a foolish person speaks without carefully thinking about what they are saying, a wise person thinks over what they want to convey before they speak a single word. It is better to be wise and silent than to be loud and foolish.