The first set of numbers on your tire size is the section width. This is the width of the tire in millimeters. For passenger vehicles and trucks, the tire section width always ends in a 5 (185, 195, 205, etc.). For motorcycles, it always ends in a zero (110, 130) and in some rare instances the section width will be in inches instead of millimeters (5, 6, etc). Trailer tires can also be in both the metric (millimeters) and numeric (inches) sizes.
The aspect ratio is the next number after the section width and slash. This number indicates the tires sidewall height from rim to tread. The larger the number the taller the sidewall and smaller numbers mean smaller sidewall.