Proverbs 17

He who has knowledge spares his words,
And a man of understanding is of a calm spirit.

Even a fool is counted wise when he holds his peace;

When he shuts his lips, he is considered perceptive. vv.27-28

Read chapter 17
Think before you speak.  That's a phrase we hear or tell ourselves often after we've said something we've regretted.  It is wise discernment to know when to speak up about a matter and when to keep quiet and hold your tongue.  This doesn't mean let someone walk all over an issue and get their way, but to use judgement to see if your words would be beneficial or hinderance to the matter at hand.  In verse ten of this same chapter says, "Rebuke is more effective for a wise man than a hundred blows on a fool."  Even a fool is thought wise that can practice self-control of his words.  When a man of understanding does speak up about a matter they carefully use their words.  
When do you have a hard time bitting your tongue?  How can you practice holding your peace about a matter till you can speak carefully and wisely?