The cavernosa are the two sinusoid areas of tissue containing the helicine arteries of the penis. Increased blood flow through these arteries gives a man an erection. They are side by side and both are needed in order to get a full erection (they "stabilize" each other architecturally). The cavernosa extends slightly into the glans; to a greater or lesser extent for each individual.
Piercings in general should not penetrate the cavernosa. A puncture of the helicine arteries will result in difficulty achieving an erection. Those that do will bleed dramatically and it is rumored that there may be difficulties in healing. This is due to the increased pressure in the arteries of the penis during erection, which may result in increased blood loss.