Meta-communication is nonverbal communication that either supplements what is being said or indicates that what is being said is not what is meant. For example, if one person asks another person, "Who is crying?" or "What is wrong?" and that person replies that nothing is wrong, obviously, the message being sent does not agree with the spoken reply. Crying is typically indicative of something wrong.
There are many reasons for meta-communication. One person may not feel comfortable with the other to tell the truth or provide the full story. Sometimes social protocol dictates that people say something very different than what they feel.
When meta-communication occurs, the words spoken are typically formalities and contain very little meaning. The term "meta-communication" was invented by Gregory Bateson in the 1970s. Bateson's idea of meta-communication is that certain body language and mannerisms combine with people's words to become codes. Effectively communicating in this way requires that all parties know and understand each others' codes.
Bateson also believed that every communication is essentially a communication within a communication, meaning that there is often more than one conversation taking place at once. There is the actual conversation -- what is being said -- and the meta-communication conversation -- what is being conveyed.