Ping is pretty much a shortened term for PRE-IGNITION. I kinda sounds like a shaker in a spray can but more dull. As above but a few things to add.
Pinging can be caused by ignition timing too far in advance, hot spots in the cylinder head (usually caused by dodgy porting), lean fuel mixtures or not enough octane rating in the fuel being used. All of these things can cause the fuel/air mix to pre-ignite.
To rectify it you need to find out which one is causing the problem and alter it. Most of the time it is poor fuel quality or ignition timing. You will find that it will ping more so when the engine is producing most of its torque so it will disappear before 6000rpm most of the time.