For others who may have the same question
1. After the fuel/air mixture is lit by the spark plug it takes some time for the mixture to burn.
2. The faster the engine turns the earlier this mixture has to be lit. (think of trap shooting - the faster the clay the more lead you need) "Lead" in this case as in "you can lead a horse to water.....not lead as in lead poisoning will kill you...In fact in some circles advance is called "lead".
3. There is an optimum time to light the mixture, too early and it wants to all get burned before the piston goes into the power stroke resulting in pre-ignition (noise), too late and power is down.
4. The distributor is what determines when the plug fires.
5. Vacuum is a method used to physically control the distributor so that the point at which the mixture is lit varies - vacuum is used with centrifigul weights in the distributor to do this.
I tried to be as simple and concise as possible.
