What is a Ring ANd Pinion on 2002 Dodge Ram 1500

Didn't know I had a problem, mechanic states it can be rebuild but recommends a new one

Asked by for the 2002 Dodge Ram 1500
Usually when a ring and pinion is failing the rear end will howl either when driving forward or coasting. The ring and pinion is located in the rear end
1 more answer
he said, Rack and Pinnion, google it