It's the bar that connects to the middle of the rear end and goes up to the transmission. NOT THE DRIVE SHAFT> It's cracked and makes a noise everytime I go over a bump. It being cracked, will it cause any major problems?

That is the torque arm. If iy breaks it will allow the rear axle to rotate, damage the driveshaft, and possibly the transmission tailshaft. also may lose control of the car. Fix it ASAP!