In terms of correspondence, "to be silent" is generally correlated with earth, the physical realm, and the body.
Ahhh, listen to that. Silence. We experience it so rarely in our hectic lives. Sometimes the best time you can give yourself is a moment of silence. It's used as a sign of respect. It's a time for thought, retrospection, and listening to what's within instead of everything without. Inner silence is a goal of many kinds of meditation. Sometimes it can be hard for us to stop and just feel - our minds seem to be going at a mile a minute all the time. Sometimes I can't get to sleep for hours because I can't make the little voice in my head be quiet (meaning that I can't stop thinking, not that I hear voices).
Magick is a time to quiet the noise in our lives - both external and internal. It's a time to allow yourself to simply BE. As some of you may know, it's hard to perform magick when your attention keeps shifting to other things. You may be calling the quarters, but you're thinking, "Did I feed the cat?" This will affect your results. In order for magick to be most effective, you need to remove all worldly distractions. Allow the voice telling you that you left the gas on and you have to pick up your dry cleaning later and you need to pay your electric bill tomorrow and . . . you get the point. That voice fades away. This kind of silent peace can seem impossible to attain, but it's not. One way to achieve it is through regular meditation. By meditating regularly, all the mundane thoughts that invade your head are allowed to play themselves out. Sooner or later, they'll run out, and you'll be left with blissful silence.
But what else could "to be silent" mean? One obvious interpretation is a common group rule: Whatever happens in the circle doesn't leave the circle. This could be anything from not mentioning group activities outside of the group to not discussing rituals and other group activities once they've been completed. This has a very functional purpose, in that not everyone who practices witchcraft wants others to know about it. It might upset their families, friends, coworkers, or any number of other people. This is why it's important to respect the privacy of the other group members.
"To be silent" can also refer to your own magickal workings. Once you've gone through the process of actually casting a spell, you need to trust in your own power and the power within the Universe that it will work. Don't spend time thinking about what will happen if it doesn't work. Don't question how you performed it and try it again, "just in case." You are an intelligent, powerful, spiritual being. If you make up your mind to do something, and focus your energy into it, you will get results. They may not be what you want, and they may not always happen when you want them to, but that's no reason to think that the magick failed. It simply took the form that was best.
Some people also interpret this as meaning that you shouldn't discuss your magick with others. I disagree with this, but you may not. I think of magick as a form of self-expression and a road to personal growth. I'm happy to share my experiences with others and learn from them in turn. I don't believe that simply speaking of your own work can in any way diminish its power or potency. On the other hand, I don't feel that everything I do is necessarily appropriate to tell others about. It's a personal choice. Although I don't have any particular superstitions about talking about my magick with others, I do feel that some of it is only for me. In this sense, I respect my own privacy, and the privacy of others by not expecting them to disclose everything they do, either.
"To be silent" reflects:
The need for peace, concentration, and silence to make magick most effective.
The respect you show to others by respecting and protecting their privacy.
Having faith in your own magick.
The respect you show yourself by respecting your own privacy.
We need to find God, and he cannot be found in noise and restlessness. God is the friend of silence. See how nature—trees, flowers, grass—grows in silence; see the stars, the moon and the sun, how they move in silence. … We need silence to be able to touch souls.
-- Mother Teresa