Extra Attacks, also commonly referred to as "Secondary Attacks", are a seperate attack the unit will attempt to perform in battle. These function very similar to primary attacks, but cannot have their Attack Power Modified positively or negatively.
Extra Attacks are however impacted by Accuracy modifiers and Efficiency modifiers (Fatigue), and can have their Initiative affected by spells/items/heroes exactly like primary attacks (in fact, if a spell reduces/increases the initiative of a primary attack it will impact the extra attack the exact same way).
Extra Attacks are commonly more powerful than primary attacks due to the fact that you can't boost their Attack Power. This also protects them from negative modification as well.