"but, speaking the truth in love, may grow up in all things into Him who is the head - Christ -" Ephesians 4:11 (NKJV)
Here in Ephesians 4 Paul is talking about spiritual growth and development. One way we can contribute to the growth and development of others is by speaking the right words, in love. It is when we speak the truth in an attitude of love with the goal of helping the other person that we will see the greatest results from our words.
So often we speak before we really consider what it is we are going to say. The problem is that once you say something you can never take those words back. As it has been said, it is better to put the mind in gear before the mouth goes into motion. I would take that a step further and say that as Christians we need to check our heart before we put our mouth into motion. Is what we have to say going to contribute in a positive way to the person or the situation?
This does not mean that we are always just saying gentle, flowery words that lack any real content or even correction. There may be times we need to help correct someone but we can do it in love and bring correction and guidance to the person instead of beating them down with words that harm instead of help. As Christians our goal should be to contribute to the overall growth and development of those around us and by speaking the truth in love we are taking a big step towards doing just that.