The Spark Table is used to adjust Spark Timing of either the front or rear cylinder. Spark Timing is relative to TDC (Top Dead Center) or bTDC (before Top Dead Center). 13 spark advance on the spark table would instruct the spark to fire 13 before TDC.

Increasing a value on the Spark Table Advances the spark firing, this means it will fire sooner.
Decreasing values on the spark table retards the timing of the spark, meaning it will fire closer to TDC.
The V-twin configuration causes the two pistons to reach TDC at different times. There is a Front Cylinder Spark Table and a Rear Cylinder Spark Table.