As Raider & 44 lite have said, it's just a ring of carbon & other crud that collects at the end of the chambers. If you DON'T clean the chambers completely, and then try to shoot 44 Magnums, they sometimes won't fit into the chamber because too much crud is in the way.
The real "problem" occurs when there is just enough crud that you can force the 44 Mag cartridges into the chambers. Then when you fire them, there isn't any room for the case neck to expand to release the bullet. This can cause the chamber pressure to become much higher than normal, potentially damaging the gun (and you). This same "problem" can happen with 357 Magnums firing 38 specials, 32 Magnums firing 32 longs etc.
All you have to do to prevent this is to keep the chambers clean. Cartridges should fall all the way into the chamber, with no pushing on them required. If your gun has really rough chambers and tends to "grab" onto the cases, then take it to a good gunsmith or send it back to the factory for a LIGHT polishing - you don't want the chambers enlarged, just a smoothing of the chamber walls. This also makes cleaning easier.