It's when the hammer spur (the part that sticks out, usually with a checkered pad, for your thumb to grab a hold of for manual cocking) is cut off.
The benefit is that it can be drawn from concealment without fear of the hammer spur getting caught on anything like a pocket, belt loop, jacket, etc. Furthermore, since it can't be fired single-action, you don't (or at least, shouldn't) have to worry about a "hair-trigger Rambo maniac" prosecution should you ever need to use the gun the self-defense.
Of course, it also means the gun cannot be manually cocked for single-action shooting, so bulls-eye shooting with such a gun would likely be out of the question. I personally think it's a good thing for a DEDICATED home-defense/carry gun, not so much for a multi-purpose gun.