This is a term describing the practice of manipulating ones fingers such that the joints (most usually the knuckle joint) produces a sharp sound, likened to cracking (also likened to popping, etc). To produce the sounds, many people will bend their fingers into unusual positions, or manipulate them in ways which are not commonly experienced in everyday use. For example, bending the fingers right back towards the back of the hand, or pulling them away from the hand.
The sound (as with the popping of cracking sounds from other joints) arises from the fact that there is air trapped in the joints. This air will re-distribute itself suddenly upon manipulation of the joint, producing a sharp sound.
The consequences of this practice upon the person performing them are unknown to the author.