Passives refer to the first skill of any hero. A passive is a skill that doesn't need a button press to be activated, and that will either give constant bonuses to the hero (like Lumburr's protect), conditional effects (like Qi's Protection aura) or general hero mechanics (like Ryoma's Naginatajutsu).
Heroes usually have 3 active skills, displayed as buttons and usually bound by mana cost and cooldown, and a passive skill. Passive skills most of the time don't have any cooldown, or have their cooldown displayed in the squares with the special effects. Passives are not affected by silence.