Think of the tire sidewall as the specification list for that specific tire. You can find all important information about the tire on the sidewall including overall diameter, overall width, tire pressure/inflation settings, load rating, speed index, and in some cases what terrain the tire was intended for. The sidewall is the smooth section that starts after the tread ends to where the tire meets the lip of the aluminum wheel.
The sidewall also serves as additional protection to the most exposed part of the tire. The raised characters, artwork or logos help keep trail debris and puncture prone obstacles at bay.