That term refers to a machine screw that has a head that resembles a block of cheese or a round of cheese. To add further mystery to the OP's inquiry, I don't know if the heads should be rounded or not, to be HC. I have made a lot of lock bolt screws out of 1/4" x 20 carriage bolts, turning them down in my electric drill or drill press with files, and then threading them 10 x 24 once they get down to 3/16" diameter. That gives bolts that have a nice big domed head and lots of length for threading.