A/R=Area to Radius ratio
Area= Area of the area
.
Radius= Radius of the outside of the "cover" (either turbine housing or compressor cover)
The a/r is the measurements between the "wheels" and the housings. It goes all around the scroll of an exhaust housing for example. It just tells you how restrictive/responsive the unit will be. Exhaust housings make power, but also can kill you with lag. A common item with the turbochargers of the past is to put larger comp covers and compressor wheels on factory turbine housings in an attempt to help with "lag". The only problem is that the factory setup is usually designed around quick, non-agressive, spool-up. On many of the older turbochargers (aka hybrid turbo's) it actually provides to be a choak point.
The smaller the number, the more restrictive, but usually, the faster spooling setup. Obviously this is comparing same type of exhaust housings. (TD05, T3, T4, T5, etc).
Compressor side is just as important. To small of a "cover" and the wheel will choak/surge..to large, and your wasting space and spool-up.
The wheel measurements (major and minor, or inducer and exducer) is what is called "trim". It simply is the ratio of the two.
Hope this helps.