A wheel spacer is a metal plate with holes in it, made to fit between a wheel hub and the back of a wheel, usually on a car or truck. The holes are there so the wheel studs can pass through the plate. Often the holes are arranged so that only a single type of spacer is needed for different stud patterns.
Wheel spacers are usually installed to correct any problems induced by incorrect wheel offset or to give a vehicle a wider track to improve stability. They can also be used to give a wheel enough clearance to fit bigger brakes.
Well made wheel spacers, fitted correctly, used to add only a small amount of track, or to correct problems caused by wheel offset should not cause any problems. Misuse of spacers, or even proper use of poorly made spacers can lead to some minor problems with wheel bearings being over-stressed, suspension geometry being thrown out of alignment, and a host of much more severe issues.
Poorly made spacers can crack and fall away from the wheel hub, allowing wheels to wobble severely, and work their way off the vehicle altogether. This can lead to obvious and catastrophic results.
Use of spacers also means that there is less of the stud protruding past the wheel, so there is less thread for the wheel nuts to grip on. This can make it possible for the nuts to work themselves loose under vibration, and fall off, which is obviously a very serious issue, leading to the aforementioned catastrophic results.