The half propshaft is used on vehicles where there are two axles at the front of the vehicle (Front wheel drive). They are so called because the run the width of half of the car and are connected from a mid-point of the vehicle called the trans-axle to each front wheel of the vehicle.
The half propshaft is connected to the front wheel and the transaxle using CV joints, which are ball and socket joints. These joints are covered with CV boots. These are rubber cases that cover the CV joints and contain a lubricant to keep the joint moving smoothly with the movement of the vehicle. Because these are made from rubber, they are prone to deterioration and cracking, which means the CV joint grease will become contaminated with dirt and debris, causing friction on the joints.
If this occurs, the CV joint will become damaged, causing movement in the half propshaft. A replacement CV joint and boot will be required. It’s often cheaper and less time-consuming to replace the whole half propshaft and the CV joint and CV boot in one go, so many mechanics will go for this option.