To my knowledge, it's a condition in the joints/bones which makes them 'clicky'. I (think I) have it, and I can click my knees, hips, knuckles, back, neck, and ankles at will, by moving them a certain way. I don't think it's an overly severe condition, but it probably varies from person to person. Also I suppose it might lead to Arthritis and other joint problems when people get older.
(I might be wrong.)