The function of a Universal Joint or U Joint is to allow for lateral and side to side movement of the driveshaft.
When your vehicle goes over bumps or over something the driveshaft needs to be able to move up and down while it turns.
The U Joint is a very important part of the driveshaft.
The Universal Joint has been around since 1844 when Edmund Morewood's 1844 patent for a metal coating machine called for a universal joint, by that name, to accommodate small alignment errors between the engine and rolling mill shafts.
However there was also a guy named Spicer in 1903 who demonstrated his new patent in a self-designed car, which did not have a troublesome chain & sprocket nor did it have chain and geared adaptions.
Spicer would then begin manufacturing of U Joints in 1904.
The U Joint allows movement of a drive shaft in up and down motion.
A U Joint also connects the driveshaft to the differential, and sometimes connects the driveshaft to the transmission or two driveshafts to each other.
The U Joint or Universal Joint is a part that is a cross shaped piece with a bearing at each end, that allows for lateral and side to side movement of the driveshaft.