This technical-sounding term refers to the relationship between the width of a tire and the height of the tire's sidewall. High-performance "low profile" tires have low aspect ratios, their sidewalls are short relative to their width. This provides extra stiffness and better high-speed handling and grip but also tends to result in a firmer (and sometimes, harsh) ride. "Taller" tires tend to provide a smoother ride and better traction in snow.