"Let no unwholesome word proceed from your mouth, but only such a word as is good for edification according to the need of the moment, so that it will give grace to those who hear. Do not grieve the Holy Spirit of God, by whom you were sealed for the day of redemption."
Words have meaning, and what we say impacts the hearer. Our speech can lead unbelievers to Christ or chase them away; they can build up a fellow Christian, or tear him down. The words we speak can please the Holy Spirit of God, or grieve Him. Before we open our mouths, we should ask ourselves, "Is what I'm about to say truthful? Is it worthwhile? Is it uplifting?" If the answer to those three questions is "no", it's probably better to remain silent.