The term aspect ratio describes the relationship of the height of a tire, compared to its width.  In the Metric tire system, this is the number following the slash, after the first three numbers.  The beginning three numbers are the width of the tire in millimeters.  For instance, with a 265/70R17, the  width is 265 millimeters, and the height is 70% of the width.  A 60 series tire would be 60% as tall as it is wide and so on.