The main attack of your unit. Every unit has one.
Primary Attacks are affected by many Attack Power Boosters/Diminishers, Accuracy Boosters/Diminishers, and Efficiency Boosters/Diminishers. These include modifiers from heroes such as Valor, Berserk, and Song. The Primary Attack's type affects unit behavior when it is on the ground. For example, an Earth Elemental might ignore a Flying stack in favor of one on the ground, if the ground stack is large enough and the elementals are also on the ground.