A 'plug chop' is when you inspect a spark plug that you have just removed. The purpose is to check that the fuel/air mixture is correct by looking at the colour of the electrodes and the end of the ceramic insulator. In a healthy engine in good tune the colour should be a light tan, brown or grey. Check out the comparison chart here:-
You can tell a lot about the set-up/state of tune of an engine by analysing the plugs and the procedure is:-
Gun the motor through the gears on a nice long straight road, preferably one that's inclining so you can labour the motor. When in top and the motor is on-song, whip in the clutch, kill the motor and coast safely to a stop.
Now have a cup of tea/smoke/lie-down for half an hour so the motor can cool, then take out the plug and inspect it's colour and condition. Be careful not to overtighten the plug when putting it back into a warm engine. Also, if you make any adjustments to the ignition and/or carburation, make sure you clean your plug(s) before doing another analysis.
This can be a very awkward job on bikes with loads of fairing panels etc, and the only alternative really is a fuel curve run taken on a dyno which is done by gas analysis - this can be quite pricey though.