A synergy stat is a player stat attribute that adds related bonuses:
Magic points adds to mana regen speed or faster cast rate.
Vitality adding life regen, Dexterity(normally affects accuracy) adds move speed,etc.

The opposite approach is to make the bonus stat item/skill based with stats being only for their effects: Strength=Damage, Dexterity=Accuracy,Vitality=Life in HP.
The 1st approach creates more value to stat bonuses, but 2nd allows more flexible/diverse equip setups.