Ephesians 4:29 (NIV), "Do not let any unwholesome talk come out of your mouths, but only what is helpful for building others up according to their needs, that it may benefit those who listen."
In the context of the Great Commission in Matthew 28:19-20, what comes out of our mouths is very important in how we communicate the Gospel to those around us. Are we spreading gossip, cussing at someone, making lewd comments about a pretty woman, or do we speak truth in a way that builds up those around us? What we say and how we say it has a lot of impact on those who hear us speak.
In a self-centric society, where it is important for each of to have a voice AND use it to speak our minds, this verse advises us to use our voices responsibly. Speak "only what is helpful for building others up according to their needs."
This does not mean tell people what they want to hear. Let your yes be yes and your no be no, but speak it in a loving and uplifting manner. Your words of encouragement can be a work of service for the Lord.