4 wheel geometry is where they get all 4 wheels pointing in the direction they should be,ie nice and straight,not sticking out/in or too much +/_ camber as to not cause any excessive wear on parts connected and most importantly tyres !! I used to go through a set of rear tyres every 6 months max until i got it done. Some of the best money Ive ever spent :) Ive had my current tyres on over a year now with no wearing issues at all :) and I dont hang about and also have the tendancy to do a little sideways action ;)