People are tempted to disregard jaw popping or clicking because it sounds like cracking your knuckles, which most people believe is harmless. In reality, though, the sounds are caused by very different phenomena, and a clicking jaw is a grave cause for concern.
Cracking your knuckles is caused by the sudden expansion of the knuckle joint, which is a simple linear joint with cartilage bathed in fluid and the entire joint encased in a capsule. When you expand the knuckle, you dramatically decrease the pressure in the joint, which causes gasses dissolved in the joint fluid to come out of solution, making the sound you hear.
In your jaw joint, the sound comes from your dislocated disk. The dislocated disk is not in the joint when your jaw is closed or only open a little bit, but when you open your jaw wide enough, the disk will slip back in, causing that popping sound. Whether or not you experience pain along with your jaw popping, you should take it seriously and seek treatment.