A universal joint or U-joint for short can also be know as a universal coupling, Cardan joint, Spicer or Hardy Spicer joint. The job of a U-joint is to transfer power from one rotating shaft to another, such as in drive shafts or axle shafts. It consists of a pair of hinges located close together, oriented at 90° to each other, connected by a cross shaft.
When u-joints go bad, that can start to make clicking or squeaking noises which are not only annoying, but mean they are on the brink of failing completely, potentially leaving you stranded on the side of the road or the trail. Don't ignore a failing u-joint, replace it.