Attack modifiers are on-hit effects that modify or give Heroes' Basic Attacks additional properties when they land on a valid target. Attack modifiers can for example cause the Basic Attack deal extra damage (flat bonuses, like Giant Killer, or charge-based crits), Splash, apply a DoT, Heal, Slow, reduce enemy's damage or even change the type of damage from Physical to Spell. Some attack modifiers can trigger on every Basic Attack while others only grant "charges" once every few attacks or after meeting some requirement, like hitting an enemy with one of your Abilities.
If the target is Evading or the attacker is Blinded, the attacker's Basic Attacks will miss the target, as a result of which the Attack modifier's effect will also not be applied. If the Attack modifier is charge-based, the charge will always be consumed on the next attack on a valid target. Thus, attacking an Evading target or while Blinded consumes the charge, but doesn't trigger the on-hit effect, effectively wasting the charge.