DRADIS is simply radar, but for outer space. Just like modern day radar, it can identify crafts based on call signs and figure out their speed and distance. The DRADIS radar on "BSG" also has a few tricks up its sleeve, like detecting nuclear explosions. Just like normal radar, it can also be tricked by stealth technology. In the show, it's a ship that's been coated in a carbon composite that ends up making it invisible to DRADIS.