Bicycle Stems connect the fork's steering tube to the handlebars and are easily changeable, allowing you to find your perfect fit and eliminate riding pain.
Steering tubes are usually the same diameter on mountain bikes and road bikes. Handlebar diameters might change from 25.4mm, 31.8mm or even super-oversized 35mm. The stem length changes how far away the handlebars are from your saddle, thus affecting how much you need to arch your back to get into the riding position.
Adjustable stems allow you to change the stem angle, raising on lowering your handlebars.