The aspect ratio of a tyre describes the height of a tyre's sidewall as a percentage of the width of the tyre

The aspect ratio of a tyre is represented by a percentage that indicates the height of the tyre, from bead to tread, in relation to the width of the tyre. For example, if the aspect ratio is 45 then it means that the height of the sidewall of a tyre is 45 percent of its width in measurement. Eg; 205/45 R16 83 V, in this case the aspect ratio of the tyre is 45 percent. 

The higher the aspect ratio of a tyre, the taller the sidewall of a tyre will be. Heavy vehicles such as such as luxury cars, buses and trucks have tyres with aspect ratios as high as 95. Tyres with high aspect ratio may compromise on handling but provide a more comfortable ride by cushioning bumps due to more air in the tyre.

On the other hand, high performance cars have tyres with a low aspect ratio. That means that these are low profile tyres with shorter and stiffer tyre walls that allow the tyre to resist cornering forces better, thereby improving handling traits of a car by reducing roll around corners. However, tyres with low aspect ratio compromise on the ride comfort of a vehicle as more vibrations are passed on from the tyre to the suspension of the vehicle. This happens as there is less air in the tyre to absorb bumps and vibrations. 

Most regular road tyres strike a balance between high and low aspect ratio to provide good handling traits for a vehicle while maintaining a comfortable ride.