The amount of ignition advance is simply the time (usually in degrees) that the spark plug ignites before the piston reaches top dead center. Most cars usually have a baseline of around 15 TDC, or the sparkplug will fire while the crankshaft moving the piston is still 15 degrees from top dead center.
The reason you want some ignition timing is because when you light the fuel/air mixture, the expansion (or 'flame wall' ) does not push down on the piston instantly. It takes a couple milliseconds to get from the sparkplug to the piston in order to push it down. Since it takes a little bit of time for the flame wall to hit the piston, it's a very common practice to ignite the mixture slightly before the piston reaches top dead center so that when the piston does reach top dead center, the flame wall hits the piston and starts pushing it down almost immediately.
Some of the benefits of proper ignition timing are smoothness of operation, better fuel efficiency, cleaner exhaust, and more power.
If you get too much ignition timing, you will hear 'pinging'. Pinging is the flame wall hitting the piston before it reaches top dead center and effectively tries to push the piston back down. This creates a very metallic rattling sound as you can imagine the insane forces of igniting the mixture without giving it anywhere to go. This can trash your engine very quickly due to the massive forces involved.
Personally, I'd start the mapping at about the lowest recommended number and see if that runs well. If you notice any hesitation or just feeling sluggish or a burble sound from the exhaust every now and then, bump the timing up a notch or two and test again until it runs right, but if you hear any metallic knocking upon acceleration, roll off the throttle and pull your timing back down a bit as soon as possible.
Hope that helps! I'm sure youtube has some excellent explanations and video animations too if you want a visual explanation.
'97 Vulcan 1500 Classic 88
Last edited by kcducttaper; 04-27-2015 at 01:49 PM.