According to Wikipedia, Meta-communication is "communicating about communication." Meta-communication is an indispensable tool for developing one's interpersonal relationships. It is important because people communicate on different levels, and one may not be aware of all the messages he is sending. The actual content of what one says is the obvious form of communication, but there are others: the context in which one says something, the tone and volume of his voice, the look in his eyes, physical posture and position, etc. Meta-communication can help one ensure that his messages are consistent. It can also help him better understand the messages sent by others.
To illustrate the idea of multiples levels of communication: imagine that, in response to a proposed resolution to a problem, one's lover says, "that's fine." If one considers this response based solely on content, then he will think that his lover his happy with the proposed resolution. But what if the words are said at three times normal speaking volume, interrupting what one is saying, and delivered with a dirty look and a grimace? The message is clearly different. This sort of thing happens all the time.
In the above example, the lover is probably intentionally sending the message that "things are not fine." This is not always the case. One may be intending to send the message that "things are fine," but is unintentionally sending contradictory messages. For instance, using the same example, assume that one's lover is truly amenable to resolving the conflict. She says things are OK, and means it, but the words are still delivered with laser eyes and in a sharp tone. Though she does not intend it, she is communicating that things are both "fine" and "not-fine" at the same time.
When this sort of miscommunication occurs, people often respond to the message opposite of the one intended. If someone is communicating that things are both "OK" and "not-OK", then the net message is that a problem still exists. Couples can continue fighting, forever, without ever identifying the source of the miscommunication. That's where meta-communication comes in.
If one is confused about contradictory messages sent by another, the proper response is not to acknowledge one of those messages and ignore the other. One can simply ask, "What are you trying to say?" (This question is so obvious and so helpful, I have no idea why people fail to use it regularly!) If she responds by identifying her intended message, then one has achieved two victories: 1) he understands what she was actually trying to communicate, and 2) he has identified a possible source of miscommunication, which he can then discuss with her. One can say something like, "when you communicate with me in this way, this is the message I get from you." In this way, two people can hammer out their immediate differences, and also learn to improve the way they communicate with each other in the future.
Meta-communication across the life of a relationship is an inductive process. Individuals must consistently maintain an awareness of how they send messages to one another, always looking out for ways to improve. There are many optional value judgments regarding the way two people communicate with each other. A distinctive form of bilateral communication is the hallmark of close interpersonal relationships. But this only happens if one puts effort into communicating about communication.