The aspect ratio of a tyre is the height of the tyre sidewall expressed as a percentage of the tyres width. To understand this more you need to recognise the meaning of the size markings on the sidewall of a tyre.
For instance a typical tyre size used today is 205 / 55 R 16 and this size can be broken down into 3 major areas:
205 - this is the width of the tyre expressed in mm.
55 - the aspect ratio, or profile of the tyre, in this case 55% of the tyre width.
16 - diameter of the rim the tyre is designed to be fitted to (in inches).
In this case the tyre sidewall height would be 113mm (205 x 55%).
It can be stated that the smaller the aspect ratio, the wider the tyre in relation to its height.
Aspect ratios will range from 80 series (eg 175/80R14) all the way down to 25 series (eg 335/25R19).