Radon is a gas in the ground, usually in mountainous areas.   Radon testing involves the radon guy coming to your house and putting a tester in.  Then about 2.5 days later he comes and picks it up. There is a scale and if the home is over a certain score then the radon needs to be mitigated.  This involves putting a tube in a crawl space and blowing a fan out a window for a couple of days. Then another radon test.