Aspect Ratio is the height of the sidewall calculated by using a percentage of the full width of the tread.

For example if you had a tyre size of 175/65R14 82 T

175 = The nominal width measured in millimeters (treaded area width)
65 = The aspect ratio (percentage height of the sidewall)

So to calculate the height of the side wall in millimeters you would calculate 65% of 175 millimeters = 113.75 millimeters of side wall.

R = Radial construction
14 = The rim/wheel size the tyre is designed to be mounted on to in Inches
82 = The maximum load rating (see chart below for conversion)
T = Maximum speed rating (see chart below for conversion)