Wikipedia describes it as:
a common, autosomal dominant, genetic follicular condition that is manifested by the appearance of rough, slightly red, bumps on the skin. It most often appears on the back and outer sides of the arm
Simply put, it's those little red bumps that some people get on their arms. Somewhere around 40% of the population suffer from it.
What happens is, the body produces too much keratin. You know keratin, it's a natural protein found in your skin and hair. So this extra protein has no where to go, so it surronds the hair follicles in the pore, producing little bumps. A lot of times, this causes ingrown hairs to form.
So what do you do if you have it?
Well, there has been some research the links a deficiency in Vitamin A to keratosis pilaris. So make sure your getting some Vitamin A.
You also will want to exfoliate the area, to help out with those ingrowns.
And of course, moisturize.