Lies. That describes one of Satan's chief characteristics. John 8:44 - He is the father of lies. That's how he approached Eve: deception. Not all lies are all lies. Get it? Some lies have truth mixed in with them to make them sound plausible. But half-truths are still lies. Should believers mimic Satan? Is he supposed to be our example? God forbid!
Another thing about this verse: notice the word "neighbors." Do you think Paul is talking about the family next door? Possible, but not necessarily the main thought he had in mind. He goes on to speak about being part of the same Body. Are your next door neighbors Christians? Maybe, maybe not. But who are your closest neighbors? Your family, and in this instance, your church family.
So, speaking the truth deals with out communication to one another as believers in Jesus. He's about to say more about what comes out of our mouths. But he starts by talking about telling the truth. Ask God to help you speak the truth clearly and find appropriate ways to help someone. Speaking the truth shouldn't be mean-spirited. There is a way to constructively criticize someone that doesn't belittle them but ministers healing to them by showing them how to improve their lives. As in verse 15, "...speak the truth in love."