In basic terms, it is an organised meeting of people often in the same line of business, creating the opportunity to get to know other business people in the area, and pitch what you have to offer.
Some good points here. Too many people go into freelancing without having a clue how to market themselves to potential clients, or even who their potential clients are. There's much more to it than setting up a cheap website and waiting for business to start rolling in. It aint gonna happen.