It is an epoxy coating (usually a greenish colour?) which is applied over the PCB after the tracks are etched, it covers the entire surface apart from the areas which are to be soldered to, i.e. the holes. While it prevents the copper tracks from oxidising in the long term, its primary purpose is to avoid track bridging during wave soldering - the wave of solder only sticks to those parts that are not covered by the mask, so only the parts that should be soldered are soldered.
Someone might correct me... but that's my understanding.
I use clear spray lacquer on my circuit boards. Works like a charm. Cheap, fast, and years later the copper is still bright and shiny and clean. Gets up out of the way of the solder nicely, too. Just spray the whole board after drilling. It won't keep a solder bridge from happening, but it slows them down--i.e. perhaps 50% of the time I can just flick the solder off without it having actually grabbed the copper.