Part of the front suspension, this device attaches the steering knuckle to the lower control arm. This is, as it's name suggests, a ball and socket joint.

The usual maintainance procedure for a ball joint involves using a grease gun to pack grease into it(the ball joint) through a special fitting called a grease zurt - which is, in fact, what most mechanic-type establishments refer to as a chassis lube. It's also customary to inspect the seals when performing this procedure.