What transmit the energy from the engine/transmission to your wheels are two axles. At both ends of each axle are rubber covers called CV (constant velocity) boots that protects the CV joints against water and dirt while keeping the special grease in. Over time these boots can be punctured by a stone thrown from another car or crack due to age. They eventually split causing all the grease to spin out, therefore allowing dirt and water to penetrate the joints. Theoretically, this will destroy any exposed CV joint in a very short period. The most common complaint for a bad outer CV joint is a clicking or clacking noise when turning corners or a vibration under acceleration indicating a bad inner joint. By catching the boot before it actually splits and ruining the joint is the key that will result in a third of the cost compared to replacement of the complete axle shaft if it is noisy.