Not to repeat what's been said but Radon is a gas that results from the decay of radium, most commonly associated with granite ledge. It has long been present in many areas and contributes to one's background radiation exposure. When homes are constructed over granite some of the gas can be trapped or concentrated, requiring venting. I would prefer a home with a venting system knowing that the long-term effects of radon in the area (it won't just be that one house after all) are being mitigated.
Your realtor should have more information about the sources and treatments for Radon. As far as the specific home you looked at, it may have been a preventative measure or one that resulted from some testing.