29er Rims are rims that support 29er rims on 29er frames. 29 inch rims are bigger than their normal counterpart of 26 inch wheels. Some of the benefits to 29″ include rolling over obstacles much more smoothly with less loss of momentum.
29er Rims have vastly better tracking through loose sand and snow. This is due both to the longer and narrower contact patch and to the reduced angle of attack on the rims. And for those without suspension at both wheels, ride quality improves a bit. Most people also report an improvement in traction too.
Also 29er Rims have less rolling resistance, giving you a smoother ride. They also have greater high speed stability, as well as much greater low-speed stability, allowing riders to clear rock gardens and switchbacks better than before.